xfwm: support for 2d pagers

[Paramjit Oberoi]

> One question: does this involve any string changes? In the shortcut editor
> perhaps? If so, there is a small problem with the string freeze. However, IMO
> the patch is nice enough to allow a break in the string freeze (sorry
> translators), especially since we're only at the start of the alpha/beta
> cycle.

The patch, as it stands, doesn't expose anything to any GUI (I wasn't
even aware of the shortcut editor), so no string changes are needed.  Of
course, ideally atleast the keyboard shortcuts should be exposed, and so
string changes would be needed.  Still, the changes are fairly minor...

> Another thing is that hidden options, like wrap workspaces, should go in
> xfwm4rc, the mcs dialog's file should never be edited manually.

I wasn't sure myself where to put that setting, since I'm not very
familiar with xfce.  It can be moved to wherever it would be
appropriate.

Also--- the functions for converting between <X,Y> coordinates and the
workspace number are fairly complicated.  I protoyped them in python
first, along with a whole bunch of test cases, and then translated them
to C.  It would be good to preserve that testing code somewhere, but I
wasn't sure where to put it.

-param