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.

arguments.c

@@ -35,7 +35,6 @@ struct args_entry {
 	RB_ENTRY(args_entry)	 entry;
 };
 
-static void			 args_set(struct args *, u_char, const char *);
 static struct args_entry	*args_find(struct args *, u_char);
 
 static int	args_cmp(struct args_entry *, struct args_entry *);
@@ -196,7 +195,7 @@ args_has(struct args *args, u_char ch)
 }
 
 /* Set argument value in the arguments tree. */
-static void
+void
 args_set(struct args *args, u_char ch, const char *value)
 {
 	struct args_entry	*entry;