Specifies whether this can own the input focus.
Widget implementations should set
focusable to true in their
init function if they
want to receive keyboard input.
Note that having
focusable be true is only one of the necessary conditions for being focusable.
A widget must also be sensitive and can-focus and not have an ancestor that is marked as not can-focus in order to receive input focus.
See grab_focus for actually setting the input focus on a widget.
whether or not this can own the input focus