Improved layout code.

Each window now has a tree of layout cells associated with it. In this tree,
each node is either a horizontal or vertical cell containing a list of other
cells running from left-to-right or top-to-bottom, or a leaf cell which is
associated with a pane.

The major functional changes are:

- panes may now be split arbitrarily both horizontally (splitw -h, C-b %) and
  vertically (splitw -v, C-b ");
- panes may be resized both horizontally and vertically (resizep -L/-R/-U/-D,
  bound to C-b left/right/up/down and C-b M-left/right/up/down);
- layouts are now applied and then may be modified by resizing or splitting
  panes, rather than being fixed and reapplied when the window is resized or
  panes are added;
- manual-vertical layout is no longer necessary, and active-only layout is gone
  (but may return in future);
- the main-pane layouts now reduce the size of the main pane to fit all panes
  if possible.

Thanks to all who tested.

resize.c

@@ -132,6 +132,7 @@ recalculate_sizes(void)
 		log_debug(
 		    "window size %u,%u (was %u,%u)", ssx, ssy, w->sx, w->sy);
 
+		layout_resize(w, ssx, ssy);
 		window_resize(w, ssx, ssy);
 
 		/*
@@ -148,6 +149,5 @@ recalculate_sizes(void)
 		}
 
 		server_redraw_window(w);
-		layout_refresh(w, 0);
 	}
 }