VSCR: Move clip info to virtual_screen

This patch moves all the related info about clip to the virtual_screen.
Now, every virtual screen will have their own clip, and the clip
is not shared between phisical screens (in Xinerama there is only one clip).

The Clip icon is shared, so is included in the global info.
VSCR: Move dock info to virtual_screen

This patch moves all the related info about dock to the virtual_screen.
Now, every virtual screen will have their own dock, and the dock
is not shared between phisical screens (in Xinerama there is only one dock).
VSCR: Move drawers info to virtual_screen

This patch moves all the related info about drawers to the virtual_screen.
Now, every virtual screen will have their own drawers, and the drawers
are not shared between phisical screens.