Fire ContextFlyoutOpened even if Flyout is null

[VisualAlf]

As I wrote in Issue #106 :

My usecase demands that I show a Window when requested by user's Right-Mouse-Click, while current solution forces me to use a Flyout instead. I could use a Flyout with a single menuitem that invokes my window but users don't like that.
A Workaround is to define an (empty) Dummy-Flyout and set the Handled Flag in the Eventhandler in order to suppress the Flyout.ShowAt().

Idea: Always fire ContextFlyoutOpened (if set), then check for Flyout not null and open it (if args.Handled is not set) .