Handle the EWMH atom _NET_WM_DESKTOP.

Handle the EWMH atom _NET_WM_DESKTOP.

We already claim _NET_WM_DESKTOP support in _NET_SUPPORTED since around 2009,
but haven't actually done anything with it. However, especially pagers like
gnome-panel rely on this property to be updated and many tools, like GTK, want
to use the corresponding client messages to make a window sticky, move it
around etc.

This patch implements full support according to the EWMH spec. This means:

* We set the property on all windows when managing it.
* We keep the property updated on all windows at all times.
* We read and respect the property upon managing a window if it was set before
  mapping the window.
* We react to client messages for it.
* We remove the property on withdrawn windows.

Note that the special value 0xFFFFFFFF, according to the spec, means that the
window shall be shown on all workspaces. We do this by making it sticky and
float it. This shows it on all workspaces at least on the output it is on.

Furthermore, the spec gives us the freedom to ignore _NET_WM_DESKTOP when
managing a window if we have good reason to. In our case, we give window
swallowing a higher priority since the user would likely expect that and we
want to keep placeholder windows only around for as long as we have to.
However, we do prioritize this property over, for example, startup
notifications.

fixes #2153
fixes #1507
fixes #938