Fixes#4364.
I havn't fully tested every single dialog and window (heck, I don't even know
how to get at half of them), and there may be some packing niggles, but this is
the bulk of the work. The Gnome 3 kiddies can close their dialogs now, anyway
:)
git-svn-id: svn://localhost/ardour2/branches/3.0@10699 d708f5d6-7413-0410-9779-e7cbd77b26cf