Powerpoint writer: Revamp slide type

This is an internal change to the Presentation type. The algebraic
datatype that used to be called `Slide` is now `Layout`, and Slide is
defined as `Slide SlideId Layout (Maybe Notes)`. Though there should
be no user-visible changes in this commit, it offers two benefits
moving forward:

1. Slides now carry their Id with them, instead of being assigned it
in deck order. This makes it easier to set up a link to, say, an
endnotes slide ahead of time.

2. This makes room for Notes slides, when we implement them.

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