Track changes in PangoContext via a serial

Track changes in PangoContext via a serial

Whenever a PangoContext or its fontmap changes we bump the
contexts serial, you can get it via pango_context_get_serial()
to see find out if the context changed since the last time and
you need to relayout.

You can also force the context to be "changed" by calling
pango_context_changed().

https://bugzilla.gnome.org/show_bug.cgi?id=340066