Rename "Default Theme Font" to reveal the actual typeface name

To improve usability and discoverability of a theme's default typeface choice, show the real typeface name in the dropdown rather than just `Default Theme Font`.

That way if the author wants to use the default theme typeface for body text (or headings), they can choose it as an option instead of just "reverting" back to the default typeface for body or headings.

See previous discussion https://teamcustomp2.wordpress.com/2013/11/12/probably-a-rare-case/ and https://wpcom.trac.automattic.com/ticket/6044

Start at the code that builds the theme font dropdown and compare how it represents the default theme typeface for body and headings. Review the linked discussion and ticket for the expected naming, then verify that the dropdown exposes the actual typeface name while preserving the default choice.

Written by the indexing model from the issue text.