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.

options-table.c

@@ -62,7 +62,7 @@ const struct options_table_entry options_table[] = {
 	  .scope = OPTIONS_TABLE_SERVER,
 	  .minimum = 1,
 	  .maximum = INT_MAX,
-	  .default_num = 20
+	  .default_num = 50
 	},
 
 	{ .name = "command-alias",
@@ -71,7 +71,9 @@ const struct options_table_entry options_table[] = {
 	  .default_str = "split-pane=split-window,"
 			 "splitp=split-window,"
 			 "server-info=show-messages -JT,"
-			 "info=show-messages -JT",
+			 "info=show-messages -JT,"
+			 "choose-window=choose-tree -w,"
+			 "choose-session=choose-tree -s",
 	  .separator = ","
 	},