xfwm: support for 2d pagers

[Paramjit Oberoi]

> It is not clear what, if any, changes are required to scroll wheel
> scrolling.

OK, now that I am more fully awake:

*  If the mouse has 2 scroll wheels: one scrolls vertically, one
   horizontally.

*  Just one wheel scrolls vertically; hotkey+wheel scrolls horizontally
   (or the other way around, depending on your inclincation).  I guess
   there can be two config vars - vert_scroll_modifier &
   horz_scroll_modifier, and one of them would be allowed to be null.

-param

PS: All this assumes that the current move to next/prev workspace
functionality is obsolete, which I am not completely convinced of.