Allow Font controls font options to be restricted to certain fonts

[vladolaru]

Via a configuration, we should be able to define recommended fonts (hand-picked) for each theme grouped by style/type (eg. Handwriting, Headings, etc).

See this: https://github.com/pixelgrade/pixelgrade-cloud/issues/11

[vladolaru]

Since we now have categories for fonts, we could restrict a font control to certain categories. Cloud fonts also come with tags.