Add ability to use the theme's default typeface for headings or body, or vice versa

If a theme uses a different typeface for body and headings, allow using the choice for the other item. For example, with the Eighties theme headings use "Varela" typeface and body text uses "Open Sans." An author might like to use Varela as body and "Open Sans" as for headings (or one or the other of the faces for both cases).

In order to use the theme's built-in typeface, the font chooser menu item will need to reveal the actual typeface name (which is a separate enhancement request).

Begin with the font chooser menu item and the theme's heading and body typeface choices described here; check how the separate request to reveal the actual typeface name affects this feature. Done means an author can select the theme's heading typeface for body text, the body typeface for headings, or either typeface for both, including the Eighties example.

Written by the indexing model from the issue text.