desktopManager.js: Ensure end rubberband selection

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 it by ending the rubberband selection if it is active and motion is detected withouth the mouse left button pressed.

Although it seems a problem only with DashToDock, it is clear that other extensions could potentially have the same problem, so it seems logical to fix it here.

Fixes #83 (closed)