Simplify the accounting of which windows we should dim by checking
the current state of windows rather than trying to track changes,
and by keeping a list of dimmed windows rather than a list of windows
with a dimmed parent. Remove windows from the list of dimmed windows
when they are destroyed.
This should fix problems where destroyed windows could end up in
the list of dimmed windows.
https://bugzilla.gnome.org/show_bug.cgi?id=644167
c6170ed751