Rewrite fonconfig cache

[abitrolly]

Looks like fc-cache causes way to much problems with an application startup times. There is at least two reports about it from Canonical.

• https://snapcraft.io/blog/the-hunt-for-rogue-time-how-we-investigated-and-solved-the-chromium-snap-slow-startup-problem
• https://snapcraft.io/blog/snap-startup-time-improvements

And the same issue seems to affecting BleachBit users (which uses Gtk, which uses fontconfig).

The solution

1. Get the specification of fontconfig cache format.
2. Make sure there a font database that uses hashes instead of path.
3. Make sure font database is shared among processes.
4. Build only a subset of using fonts.
5. Build font cache incremementally.
6. Expose stats about time that it takes to discover, build and store fonts. Both realtime and historical.
7. Get at least one alternative implementation in Go or Python, and cover interop with tests.
8. Write separate love story for google-noto font bomb.

[abitrolly]

Links:

• https://github.com/marguerite/fonts-config-ng/ - some code in Go

[abitrolly]

See also bleachbit/bleachbit#1072