Implement rg_move_prev/rg_move_next to move workspaces across regions/screens

adds rg_move_next (M-c for "cycle") and rg_move_prev (M-S-c).

It's a convenience function to move a workspace to the next/previous
screen, having the effect of swapping screen contents with the
next/previous screen.

It simply reuses switchws() to do the work.

I was tired of hunting down the workspace number of the screen.

Looks like an alternate implementation of #66.
Existing "discussion": http://thread.gmane.org/gmane.comp.window-managers.spectrwm/37