Rewrite of choose mode, both to simplify and tidy the code and to add

some modern features.

Now the common code is in mode-tree.c, which provides an API used by the
three modes now separated into window-{buffer,client,tree}.c. Buffer
mode shows buffers, client mode clients and tree mode a tree of
sessions, windows and panes.

Each mode has a common set of key bindings plus a few that are specific
to the mode. Other changes are:

- each mode has a preview pane: for buffers this is the buffer content
  (very useful), for others it is a preview of the pane;

- items may be sorted in different ways ('O' key);

- multiple items may be tagged and an operation applied to all of them
  (for example, to delete multiple buffers at once);

- in tree mode a command may be run on the selected item (session,
  window, pane) or on tagged items (key ':');

- displayed items may be filtered in tree mode by using a format (this
  is used to implement find-window) (key 'f');

- the custom format (-F) for the display is no longer available;

- shortcut keys change from 0-9, a-z, A-Z which was always a bit weird
  with keys used for other uses to 0-9, M-a to M-z.

Now that the code is simpler, other improvements will come later.

Primary key bindings for each mode are documented under the commands in
the man page (choose-buffer, choose-client, choose-tree).

Parts written by Thomas Adam.

cmd-split-window.c

@@ -134,7 +134,7 @@ cmd_split_window_exec(struct cmd *self, struct cmdq_item *item)
 		goto error;
 	}
 	new_wp = window_add_pane(w, wp, args_has(args, 'b'), hlimit);
-	layout_assign_pane(lc, new_wp);
+	layout_make_leaf(lc, new_wp);
 
 	path = NULL;
 	if (item->client != NULL && item->client->session == NULL)
@@ -152,6 +152,7 @@ cmd_split_window_exec(struct cmd *self, struct cmdq_item *item)
 	}
 	environ_free(env);
 
+	layout_fix_panes(w, w->sx, w->sy);
 	server_redraw_window(w);
 
 	if (!args_has(args, 'd')) {