GdkFrameTimings: strip down to a minimal public API

GdkFrameTimings: strip down to a minimal public API

Since we're not exporting the ability to create your own frame
clock for now, remove the setters for GdkFrameTimings fields.
Also remove all setters and getters for fields that are more
about implementation than about quantities that are meaningful
to the applcation and just access the fields directly within
GDK.