added:
pango_fc_font_map_set_default_substitute
pango_fc_font_map_default_substitute_changed

deprecated:
pango_ft2_font_map_set_default_substitute
pango_ft2_font_map_changed
pango_xft_font_map_set_default_substitute
pango_xft_font_map_changed

Now PangoCairoFcFontMap will call what is passed to
pango_fc_font_map_set_default_substitute when it is time.

The deprecated functions make calls to the parent (FC) class now.
The user-supplied callbacks are executed in the exact same places as
before.