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-respawn-window.c

@@ -70,6 +70,7 @@ cmd_respawn_window_exec(struct cmd *self, struct cmd_ctx *ctx)
 
 	wp = TAILQ_FIRST(&w->panes);
 	TAILQ_REMOVE(&w->panes, wp, entry);
+	layout_free(w);
  	window_destroy_panes(w);
 	TAILQ_INSERT_HEAD(&w->panes, wp, entry);
 	window_pane_resize(wp, w->sx, w->sy);
@@ -78,6 +79,7 @@ cmd_respawn_window_exec(struct cmd *self, struct cmd_ctx *ctx)
 		xfree(cause);
 		return (-1);
 	}
+	layout_init(w);
 	screen_reinit(&wp->base);
 
 	recalculate_sizes();