tabbed: UnmapNotify patch

Hi,

Simple patch for tabbed that prevents tabbed from killing a client that it
should no longer be managing due to an UnmapNotify event.

Add support for UnmapNotify; allows a window to map and unmap without
needing it to be destroyed.

Thanks,
Okan

Signed-off-by: Christoph Lohmann <20h@r-36.net>