Fonts api with Themes integrated

Fixes:
https://github.com/adobe/brackets/issues/8425
https://github.com/adobe/brackets/issues/8381

References:
https://github.com/adobe/brackets/pull/8305

I remember now.  I use that function in the extension because I have a reset button in the settings dialog.  I am thinking that something like might actually be useful here too.  What do you think?