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-previous-layout.c

@@ -44,12 +44,13 @@ cmd_previous_layout_exec(struct cmd *self, struct cmd_ctx *ctx)
 {
 	struct cmd_target_data	*data = self->data;
 	struct winlink		*wl;
-
+	u_int			 layout;
+	
 	if ((wl = cmd_find_window(ctx, data->target, NULL)) == NULL)
 		return (-1);
 
-	layout_previous(wl->window);
-	ctx->info(ctx, "layout now: %s", layout_name(wl->window));
+	layout = layout_set_previous(wl->window);
+	ctx->info(ctx, "arranging in: %s", layout_set_name(layout));
 
 	return (0);
 }