Rewrite of choose mode, both to simplify and tidy the code and to add

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.