This fixes the bug which caused floating windows to be visible even when
switching to a different workspace.
Instead of ignoring a specific sequence, we now set an ignore_unmap counter for
each container. (So, should containers be closed too early or stay open even if
they should be closed, we probably need to have a closer look at the counter.
At the moment, it is increased by one on reparenting and unmapping (for
workspace changes) and decremented by one on each UnmapNotify event).
This system is better because a sequence does not describe a single unmap or
reparent request but a request to X11 on the network layer -- which can contain
multiple requests.
