GdkWindow: make the frame clock an inherent property of the window

GdkWindow: make the frame clock an inherent property of the window

Instead of making the frame clock a settable property of a window, make
toplevel windows inherently have a frame clock when created (getting
rid of the default frame clock.) We need to create or destroy frame
clocks when reparenting a window to be a toplevel, or to not be a
toplevel, but otherwise the frame clock for a window is immutable.