xfwm: support for 2d pagers

[Paramjit Oberoi]

> What's the reason for having prev/next besides left/right/up/down? In default
> operation left/right will simply be the same as prev/next.

Suppose you have a 2d pager with 2 rows & 4 cols:

+---+---+---+---+
| 0 | 1 | 2 | 3 |
+---+---+---+---+
| 4 | 5 | 6 | 7 |
+---+---+---+---+

Say the current workspace is 3.  Next always goes to workspace 4.  Move
right either stays at 3 or goes to 0 depending on whether wrap_layout is
disabled or enabled.

So, is there any benefit to next/prev?  I'm not sure.  They are only
useful if you think of your workspaces as simultaneously being in a grid
as well as being in a list.  It's probably possible to come up with
cases where for half a day a grid makes sense, and for the other half a
list makes sense, and you don't want to keep modifying your pager
geometry constantly...

My opinion is that next/prev should be left as they are in the absence
of strong opinions against them.

-param