currently, the windows-menu is repopulated whenever anything changes,
that is, upon window creation, deletion, MDI-/un-MDI-fying, and window
title change. i hope this covers everything.
alternatively, we could connect to QMenu::aboutToShow (see
rebuild the menu whenever it gets displayed.
the current solution may miss some changes, while it only is called when
there really is a change. the aboutToShow-way cannot miss anything, but
will be executed probably too often...
what do you think we should do?
don't change it at all?
change it when we find out that it misses changes ("if it ain't broken,
don't fix it")?
or immediately use aboutToShow?