arx-install-data: Fix patch filename check

Fixes a bogus warning about the patch filename being unexpected.
Move implementation details out of FontCache.h
Improve error message when missing the debug font
Move g_FTLibrary into FontCache::Impl class
Add m_ prefix to FontCache::Impl members
Fix FontFile cleanup when a font could not be loaded

When a font file could not be loaded for a particular size, but was
already loaded for other sizes, the FontFile entry was erroneously
erased, leading memory leaks and possibly other fun stuff.
Update the arx man page with the new options
Add a man page for the arx-install-data script