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.

Makefile

@@ -10,8 +10,6 @@ SRCS=	alerts.c \
 	cmd-bind-key.c \
 	cmd-break-pane.c \
 	cmd-capture-pane.c \
-	cmd-choose-buffer.c \
-	cmd-choose-client.c \
 	cmd-choose-tree.c \
 	cmd-command-prompt.c \
 	cmd-confirm-before.c \
@@ -20,6 +18,7 @@ SRCS=	alerts.c \
 	cmd-display-message.c \
 	cmd-display-panes.c \
 	cmd-find-window.c \
+	cmd-find.c \
 	cmd-if-shell.c \
 	cmd-join-pane.c \
 	cmd-kill-pane.c \
@@ -40,6 +39,7 @@ SRCS=	alerts.c \
 	cmd-new-window.c \
 	cmd-paste-buffer.c \
 	cmd-pipe-pane.c \
+	cmd-queue.c \
 	cmd-refresh-client.c \
 	cmd-rename-session.c \
 	cmd-rename-window.c \
@@ -69,11 +69,9 @@ SRCS=	alerts.c \
 	cmd-unbind-key.c \
 	cmd-wait-for.c \
 	cmd.c \
-	cmd-find.c \
-	cmd-queue.c \
 	colour.c \
-	control.c \
 	control-notify.c \
+	control.c \
 	environ.c \
 	format.c \
 	grid-view.c \
@@ -88,6 +86,7 @@ SRCS=	alerts.c \
 	layout-set.c \
 	layout.c \
 	log.c \
+	mode-tree.c \
 	names.c \
 	notify.c \
 	options-table.c \
@@ -112,9 +111,11 @@ SRCS=	alerts.c \
 	tty-term.c \
 	tty.c \
 	utf8.c \
-	window-choose.c \
+	window-buffer.c \
+	window-client.c \
 	window-clock.c \
 	window-copy.c \
+	window-tree.c \
 	window.c \
 	xmalloc.c \
 	xterm-keys.c