Make possible for apps to disallow the navigation bar slide gesture

On narrow screens a slide gesture can be used to open or close the
navigation bar. However that gesture could conflict at times with the
gestures used by certain apps (for example, if the right sidebar is open
the user may expect to close it by dragging it to the right, but that
could open the navigation bar instead depending on how the events are
handled). This commit makes possible for apps to disallow and allow
again that slide gesture.

In any case, note that applications can only disallow the gesture,
but they can not enable it. That is, they can prevent the gesture from
being used on narrow screens, but they can not make the gesture work on
wide screens; they are always limited by the base rules set by the core.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>