desktopManager.js: workaround to reset the rubberband state

With DashToDock enabled, if the user starts a rubberband selection and accidentally ends it over the dock, the rubberband remains active. But pressing the mouse button in the desktop, although ends the selection, leaves the extension in an invalid state: not only it doesn't release naturally the focus (only when the user presses the left button in a window), but also the rubberband selection is still active in a "ghost mode".

This patch fixes this by ensuring that any pending rubberband is ended before starting another one. So, if the user ends a rubberband over DashToDock (and thus the rubberband isn't terminated), just clicking on the desktop will end the rubberband selection, leaving it in the right state and allowing the user to try again.

This patch is only a workaround until we discover what happens with get_status_full() under Xorg, and whether get_status() is safe or not. It is an alternative to !103 (merged) , although this one is only a workaround, while !103 (merged) is a true fix.