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.

cmd-choose-window.c

@@ -76,8 +76,8 @@ cmd_choose_window_exec(struct cmd *self, struct cmd_ctx *ctx)
 		idx++;
 
 		window_choose_add(wl->window->active,
-		    wm->idx, "%3d: %s [%ux%u %s] (%u panes)", wm->idx, w->name,
-		    w->sx, w->sy, layout_name(w), window_count_panes(w));
+		    wm->idx, "%3d: %s [%ux%u] (%u panes)",
+		    wm->idx, w->name, w->sx, w->sy, window_count_panes(w));
 	}
 
 	cdata = xmalloc(sizeof *cdata);