Render tree before destroying X11 containers upon unmap (Thanks Merovius)

Render tree before destroying X11 containers upon unmap (Thanks Merovius)

When an X11 window is closed (say, urxvt), i3 gets an UnmapNotify event
and destroys (DestroyWindow) the window decorations. Before this commit,
the DestroyWindow call was sent immediately.

This lead to a situation where — due to the DestroyNotify — EnterNotify
events were generated that would cause the focus to be set to the
underlying window.

With this commit, i3 first renders the tree and pushes changes to X11
before calling DestroyWindow. Therefore, the surrounding containers will
take up any space that was freed by the window which was closed and no
EnterNotify will be generated.

fixes #660