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.

window-copy.c

@@ -27,7 +27,8 @@
 static const char *window_copy_key_table(struct window_pane *);
 static void	window_copy_command(struct window_pane *, struct client *,
 		    struct session *, struct args *, struct mouse_event *);
-static struct screen *window_copy_init(struct window_pane *);
+static struct screen *window_copy_init(struct window_pane *,
+		    struct cmd_find_state *, struct args *);
 static void	window_copy_free(struct window_pane *);
 static int	window_copy_pagedown(struct window_pane *, int);
 static void	window_copy_next_paragraph(struct window_pane *);
@@ -187,7 +188,8 @@ struct window_copy_mode_data {
 };
 
 static struct screen *
-window_copy_init(struct window_pane *wp)
+window_copy_init(struct window_pane *wp, __unused struct cmd_find_state *fs,
+    __unused struct args *args)
 {
 	struct window_copy_mode_data	*data;
 	struct screen			*s;