Bugfix: Unmap windows before reparenting them to the root window (fixes Mathematica) (Thanks psychon)

Some apps such as Mathematica send a synthetic UnmapNotify event without
properly unmapping their window first. This change makes sure that
happens and fixes an annoying bug with Mathematica where some unmanaged
windows would stay around, but you couldn’t do anything with them.

Thanks to psychon (current awesome maintainer) for helping with the
diagnosis!

fixes #787
Merge branch 'fix-unmap'