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.

Makefile

@@ -27,7 +27,7 @@ SRCS=	attributes.c buffer-poll.c buffer.c cfg.c client-fn.c \
 	cmd-switch-client.c cmd-unbind-key.c cmd-unlink-window.c \
 	cmd-up-pane.c cmd-display-message.c cmd.c \
 	colour.c grid-view.c grid.c input-keys.c \
-	input.c key-bindings.c key-string.c layout-manual.c layout.c log.c \
+	input.c key-bindings.c key-string.c layout-set.c layout.c log.c \
 	mode-key.c names.c options-cmd.c options.c paste.c procname.c \
 	resize.c screen-redraw.c screen-write.c screen.c server-fn.c \
 	server-msg.c server.c session.c status.c tmux.c tty-keys.c tty-term.c \