Add the ability to freeze a frame clock, which pauses its operation,
then thaw it again later to resume.

Initially this is used to implement freezing updates when we are
waiting for ConfigureNotify in response to changing the size of
a toplevel.

We need a per-window clock for this to work properly, so add that
for the X11 backend.

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