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.
With the polygon, when using pidgin, having the buddy list in the middle of the
screen, 200 px width, full screen high, then opening the manage accounts
window, the decorations of the buddy list were visible on the accounts window.
The implementation works like this:
Containers can have a 'sticky-group' attribute. Imagine two different
containers (on two different workspaces) which have the same sticky-group.
Now you open a window in the first container. When you switch to the
other workspace, the window will be re-assigned to the other container.
An obvious problem which is not covered with the code at the moment is
having two containers with the same sticky-group visible at the same time.
