Gestures for Flyout barrier

[bdlukaa]

Is your feature request related to a problem? Please describe.
Clicking the right mouse button to reposition the popup context menu is a habit of desktop users. It is currently not possible to detect gestures on the barrier.

Describe the solution you'd like
An interface, or callback, that allows devs to detect gestures on the Flyout barrier. Something like thew following:

controller.showFlyout(
  barrierRecognizer: TapGestureRecognizer()..onTap = () => print('Tapped Flyout barrier'),
  ...,
);

Describe alternatives you've considered
An workaround can be use a Flyout that fills the entire screen and position the barrier and content manually. This takes more time to do, but still work.

Additional context
#975