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-list-windows.c

@@ -78,9 +78,8 @@ cmd_list_windows_exec(struct cmd *self, struct cmd_ctx *ctx)
 			else
 				name = "unknown";
 			ctx->print(ctx,
-			    "     %s [%ux%u %s] [history %u/%u, %llu bytes]",
-			    name, wp->sx, wp->sy, layout_name(w), gd->hsize,
-			    gd->hlimit, size);
+			    "     %s [%ux%u] [history %u/%u, %llu bytes]",
+			    name, wp->sx, wp->sy, gd->hsize, gd->hlimit, size);
 		}
 	}