We shouldn't render a window before it is mapped (obviously), but we
render all windows in the view list. Hence, only insert the window once
it is mapped.
We could run into the case where a window is destroyed without being in
the window list, so we now track unmapping again and remove windows from
the list when they get unmapped.
This makes Cage much easier to maintain. Not only is it easier where to
look and to maintain a mental model of the code, there is also more
encapsulation, better abstractions and better extendability.
