Add gdk_frame_timings_get/set_slept_before()

Add gdk_frame_timings_get/set_slept_before()

Add functions that tell us whether the main loop slept before we drew
a frame. Blocking with the frame clock frozen doesn't count as sleeping.
We'll use this to advertise to the compositor whether we
are drawing as fast as possible (and it should do the same) or timing
frames carefully (and it should do the same.)

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