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

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