[script] Find and interact with windows

This patch adds mechanisms to find and interact with windows to the
scripting API:

 - workspace.windowAt provides a way to list the windows at a given
   location on the screen.
 - workspace.stackingOrder provides a list of all windows in the order
   they are stacked on the screen.
 - workspace.raiseWindow provides a mechanism to bring a given window to
   the top of that list, making it visible above all others.

Signed-off-by: Antonio Russo <aerusso@aerusso.net>