xfwm: support for 2d pagers

[Paramjit Oberoi]

> i'm curious as to why you made that choice.  why not leave the alt+ctrl 
> combos the same, and add up and down for navigation?  i think the 
> navigation shortcuts are used much more frequently than the move window 
> shortcuts.  seems like it would make sense to learn new move window 
> shortcuts than navigation.

Just to clarify: the keybindings I listed are not suggested defaults.
They are just what I have been using for a long time.  The defaults
should be picked so that they blend in with the already existing
shortcuts.

-param