CoreText backend: implement font fallbacks

CoreText backend: implement font fallbacks

We implement font fallbacks by making use of
CTFontCopyDefaultCascadeList, a public, non-exposed, symbol in the
CoreText library.  We might want to handle this differently.

The code now also uses a PangoCoreTextFontsetKey and
PangoCoreTextFontKey, similar to the font config backend. This has also
helped to clean up the code in general.

This patch contains two fixes by Michael Natterer:
 1. Fix a bug in pango_core_text_font_key_equal() where the return value
of memcmp() was wrongly interpreted.
 2. Cache PangoCoverage in PangoCoreTextFont.