Clarify key_handle*

Clarify key_handle*

key_handle should be used most often.
  It uses the focus given, but moves 'home' out to a leaf and
    searches in from there.
key_handle_filter should be used only when calling a function with
    the same name as self.  It uses focus and home as given and
    only searches home towards root
key_handle_focus should be used when simulting keyboard input.  It
    moves focus out to a leaf, and then sets home from there.
key_handle_xy should be used when simulatiing mouse input.  it follows
    focus out to a root tracking x,y and starts home from there.

key_handle_focus and key_handle_xy also pick-up ->point if it is set on
any pane on the way out.

Signed-off-by: NeilBrown <neil@brown.name>