All headings now have a uniform color.
Level-1 headings no longer set `w:themeShade="B5"`.
Level-2 headings are now 14 point rather than 16 point.
Level-3 headings are now 12 point rather than 14 point.
Level-4 headings are italic rather than bold.
Closes#5820.
Instead of just "Hello, world", the document now contains
exemplars of most of the styles that have an effect on pandoc
documents.
This makes it easier to see the effect of style changes.
Closes#4175.
Figures with empty captions use style "Figure"
Figures with nonempty captions use style "Figure with Caption", which is
based on "Figure", and additionally has keepNext set.
