pptx: Add support for more layouts

Until now, the pptx writer only supported four slide layouts: “Title
Slide” (used for the automatically generated metadata slide), “Section
Header” (used for headings above the slide level), “Two Column” (used
when there’s a columns div containing at least two column divs), and
“Title and Content” (used for all other slides).

This commit adds support for three more layouts: Comparison, Content
with Caption, and Blank.

- Support “Comparison” slide layout

  This layout is used when a slide contains at least two columns, at
  least one of which contains some text followed by some non-text (e.g.
  an image or table). The text in each column is inserted into the
  “body” placeholder for that column, and the non-text is inserted into
  the ObjType placeholder. Any extra content after the non-text is
  overlaid on top of the preceding content, rather than dropping it
  completely (as currently happens for the two-column layout).

  + Accept straightforward test changes

    Adding the new layout means the “-deleted-layouts” tests have an
    additional layout added to the master and master rels.

  + Add new tests for the comparison layout
  + Add new tests to pandoc.cabal

- Support “Content with Caption” slide layout

  This layout is used when a slide’s body contains some text, followed by
  non-text (e.g. and image or a table). Before now, in this case the image
  or table would break onto a new slide: to get that output again, users
  can add a horizontal rule before the image or table.

  + Accept straightforward tests

    The “-deleted-layouts” tests all have an extra layout and relationship
    in the master for the Content with Caption layout.

  + Accept remove-empty-slides test

    Empty slides are still removed, but the Content with Caption layout is
    now used.

  + Change slide-level-0/h1-h2-with-text description

    This test now triggers the content with caption layout, giving a
    different (but still correct) result.

  + Add new tests for the new layout
  + Add new tests to the cabal file

- Support “Blank” slide layout

  This layout is used when a slide contains only blank content (e.g.
  non-breaking spaces). No content is inserted into any placeholders in
  the layout.

  Fixes #5097.

  + Accept straightforward test changes

    Blank layout now copied over from reference doc as well, when
    layouts have been deleted.

  + Add some new tests

    A slide should use the blank layout if:

    - It contains only speaker notes
    - It contains only an empty heading with a body of nbsps
    - It contains only a heading containing only nbsps

- Change ContentType -> Placeholder

  This type was starting to have a constructor for each placeholder on
  each slide (e.g. `ComparisonUpperLeftContent`). I’ve changed it
  instead to identify a placeholder by type and index, as I think that’s
  clearer and less redundant.

- Describe layout-choosing logic in manual

1 files changed, 49 insertions, 3 deletions

diff --git a/MANUAL.txt b/MANUAL.txt
index f81f4c70b..afdd66ddd 100644
--- a/MANUAL.txt
+++ b/MANUAL.txt
@@ -1182,11 +1182,15 @@ header when requesting a document from a URL:
         - Title and Content
         - Section Header
         - Two Content
+        - Comparison
+        - Content with Caption
+        - Blank
 
         For each name, the first layout found with that name will be used.
         If no layout is found with one of the names, pandoc will output a
         warning and use the layout with that name from the default reference
-        doc instead.
+        doc instead. (How these layouts are used is described in [PowerPoint
+        layout choice](#powerpoint-layout-choice).)
 
         All templates included with a recent version of MS PowerPoint
         will fit these criteria. (You can click on `Layout` under the
@@ -1195,8 +1199,8 @@ header when requesting a document from a URL:
         You can also modify the default `reference.pptx`: first run
         `pandoc -o custom-reference.pptx --print-default-data-file
         reference.pptx`, and then modify `custom-reference.pptx`
-        in MS PowerPoint (pandoc will use the first four layout
-        slides, as mentioned above).
+        in MS PowerPoint (pandoc will use the layouts with the names
+        listed above).
 
 `--epub-cover-image=`*FILE*
 
@@ -5833,6 +5837,48 @@ you use deeper nesting of section levels with reveal.js unless you set
 `--slide-level=0` (which lets reveal.js produce a one-dimensional layout
 and only interprets horizontal rules as slide boundaries).
 
+### PowerPoint layout choice
+
+When creating slides, the pptx writer chooses from a number of pre-defined
+layouts, based on the content of the slide:
+
+Title Slide
+:   This layout is used for the initial slide, which is generated and
+    filled from the metadata fields `date`, `author`, and `title`, if
+    they are present.
+
+Section Header
+:   This layout is used for what pandoc calls “title slides”, i.e.
+    slides which start with a header which is above the slide level in
+    the hierarchy.
+
+Two Content
+:   This layout is used for two-column slides, i.e. slides containing a
+    div with class `columns` which contains at least two divs with class
+    `column`.
+
+Comparison
+:   This layout is used instead of “Two Content” for any two-column
+    slides in which at least one column contains text followed by
+    non-text (e.g. an image or a table).
+
+Content with Caption
+:   This layout is used for any non-two-column slides which contain text
+    followed by non-text (e.g. an image or a table).
+
+Blank
+:   This layout is used for any slides which only contain blank content,
+    e.g. a slide containing only speaker notes, or a slide containing
+    only a non-breaking space.
+
+Title and Content
+:   This layout is used for all slides which do not match the criteria
+    for another layout.
+
+These layouts are chosen from the default pptx reference doc included with
+pandoc, unless an alternative reference doc is specified using
+`--reference-doc`.
+
 ## Incremental lists
 
 By default, these writers produce lists that display "all at once."