It works exactly like AeroSnap.
Except for shift+win+left/right, which is left for AeroSnap
to handle (AeroSnap takes action before we get the message,
so there's no way for us to override it).
The only thing that doesn't work is shift+win+left/right on
a maximized window, for reasons unknown at the moment.

This only implements winkey+stuff behaviour of AeroSnap,
not the drag-to-the-edge-and-something-funny-happens one.

https://bugzilla.gnome.org/show_bug.cgi?id=763013