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-queue.c

@@ -428,7 +428,8 @@ cmdq_print(struct cmdq_item *item, const char *fmt, ...)
 		w = c->session->curw->window;
 		if (w->active->mode != &window_copy_mode) {
 			window_pane_reset_mode(w->active);
-			window_pane_set_mode(w->active, &window_copy_mode);
+			window_pane_set_mode(w->active, &window_copy_mode, NULL,
+			    NULL);
 			window_copy_init_for_output(w->active);
 		}
 		window_copy_vadd(w->active, fmt, ap);