[RP] initial split windows

[shawn]

Soooo, the day has come: split windows. good or bad? Well I'm not
sure, but I've added it anyway :)

its got some bugs 'n' problems but it works more or less
correctly. You'll notice it behaves more like emacs windows and less
like screen splits.

There is a problem with transient windows if they are bigger than the
frame they are supposed to fit into...bloody transients...