better desktop integration: edge of screen movement

[Antoine Martin]

We should be able to re-use some of the techniques from ​x2vnc and ​x2x to detect when the window/mouse is over the edge of the screen and use this as a trigger to send the application to the machine defined as being attached to that edge.

Difficulties:

• Integration with the window manager: how do we find if there is a window being dragged easily? (in python and without libX11/win32 code if possible...) - maybe defining a compiz extension would be easier?
• UI for defining which machines are attached to which edges... and how to deal with machines coming and going in a consistent manner.