window: Allow minimizing windows which don't advertise support for it

Wine removes the minimize func from its Motif hints on full-screen
windows, because, as the Win32 API literally says, the minimize button
is indeed not visible on full-screen windows.

Given that this code was added to prevent minimizing a panel by
accident, I don't necessarily think that it's relevant anymore.

https://bugzilla.gnome.org/show_bug.cgi?id=758186