Allow resizing or enlarging windows

[adehnert]

In certain circumstances (eg, showing the bindings for the compose window), you can get really small windows. Some mechanism for making it bigger would be nice.

[kcr]

I, uh, totally thought I'd implemented C-x ^. I should implement C-x + while I'm at it.

[kcr]

enlarge window (C-x ^) in 990973e, balance windows (C-x +) in b19b1fe. I'm leaving this open because for further "popups" from popups like the compose window, under some circumstances you want them to steal real-estate from the adjacent window instead of subdividing.