The mapping and unmapping of the X11 stage implementation is
a bit bong. It's asynchronous, for starters, when it really
can avoid it by tracking the state internally.
The ordering of the map/unmap sequence is also broken with
respect to the resizing.
By tracking the state internally into StageX11 we can safely
remove the MapNotify and UnmapNotify X event handling.
In theory, we should use _NET_WM_STATE a lot more, and reuse
the X11 state flags for fullscreening as well.
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