Build a font group by doing private matching according to CSS spec

[burg]

FontContext::create_font_group needs to consult the Font list, available families, and so on to map a FontStyle to concrete Font instances.

Tasks

• (A) Implement FontList and FontListHandle, which are the generic and platform-specific machinery for populating a list of font families.
• (B) Implement population of FontFamily and FontEntry.
• (C) Implement basic font matching for 1-face and 4-face font families.
• (D) Hook up the matching API to create_font_group.
• (E) Cache FontGroup instances.

[burg]

It seems pretty clear that FontContext should do the heavy lifting here, since it could cache FontGroup instances, and almost all of the data needed to do matching is owned by FontContext.

[burg]

Initial work done. Followup bugs: #194, #193, #192, #190, #189, #188, #186.