Add an UPDATE phase and GdkFrameClockTarget, use for GtkStyleContext

Add an UPDATE phase and GdkFrameClockTarget, use for GtkStyleContext

Switch GtkStyleContext to using GdkFrameClock. To do this, add a new
UPDATE phase to GdkFrameClock.

Add a GdkFrameClockTarget interface with a single set_clock() method,
and use this to deal with the fact that GtkWidget only has a frame
clock when realized.

https://bugzilla.gnome.org/show_bug.cgi?id=685460