[Enhancement] Popup should be able to open or close programtically

[haavamoa]

Summary

The consumer of the popup should be able to open or close the popup programticall. This can be achieved by using the attached property in the view, but it should also be able to do it by a bindable property.

API Changes

Concider adding a boolean IsOpen to the PopupBehaviour.

• This should be bindable with mode : TwoWay
• It should be set to true when the user clicks the view element that owns the behaviour, so our consumers can programtically know when the user has clicked it.
  -It should be set to false when the user closes the popup (either through clicking outside or by clicking a view with Popup.CloseOnClick.)
• It should react to property changed and open / close the popup programtically.