WindowOverlay: animate the close button together with the border

WindowOverlay: animate the close button together with the border

Since the introduction of overlay hover borders, there has a been
a timing disconnection between hiding the border and button, and
this creates noise and reduces the effect of the window+overlay
as a single unit.
Solve that by animating the close button too, so that the two actors
are shown and hidden always at the same time.
Also, consolidate the code to make it clear to future authors that
those two items need to stay coordinated.

https://bugzilla.gnome.org/show_bug.cgi?id=688966