GtkFontChooserWidget: Listen for fontconfig changes

GtkFontChooserWidget: Listen for fontconfig changes

We were relying on indirectly getting notify when fontconfig
configuration changes, by GtkSettings translating the timestamp
change into a style-invalidation, which gets fed through the
css invalidation machinery. That machinery has gotten good enough
at optimizing away redundant changes that it no longer emits
::style-updated in this case.

So, instead make the font chooser listen directly to what it
cares about: the fontconfig change notification from GtkSettings.