Add a feature switch to the desktop when cycling windows

Sometimes it is useful to be able to switch to the desktop when cycling windows. For example, in the case of a full-screen application.

This patch added the desktop to the list of windows when cycling windows. The desktop is always the last item on the list. A setting to disable this behavior has also been added. The HAS_STICK flag is preserved for the desktop window so that this window always has the current desktop number.