Fix meta_screen_get_mouse_window when desktop is shown

Fixes https://github.com/linuxmint/Cinnamon/issues/2559 and https://github.com/linuxmint/Cinnamon/issues/2877

The problem is that when the desktop is "shown", the function ```meta_screen_get_mouse_window``` returns the window that would have the mouse if the desktop were hidden, but it should instead only return the desktop itself. This pull request fixes it to do that.