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.

server.c

@@ -840,7 +840,9 @@ server_handle_client(struct client *c)
 
 	/* Ensure cursor position and mode settings. */
 	status = options_get_number(&c->session->options, "status");
-	if (wp->yoff + s->cy < c->tty.sy - status)
+	if (!window_pane_visible(wp) || wp->yoff + s->cy >= c->tty.sy - status)
+		tty_cursor(&c->tty, 0, 0, 0, 0);
+	else
 		tty_cursor(&c->tty, s->cx, s->cy, wp->xoff, wp->yoff);
 
 	mode = s->mode;
@@ -1072,9 +1074,9 @@ server_check_window(struct window *w)
 		 * pane dies).
 		 */
 		if (wp->fd == -1 && !flag) {
+			layout_close_pane(wp);
 			window_remove_pane(w, wp);
 			server_redraw_window(w);
-			layout_refresh(w, 0);
 		} else 
 			destroyed = 0;
 		wp = wq;