Release 2.2.0 - Dialog Factory Changes

This is the first breaking change in awhile, although it only affects the Dialog factory. JavaFxObservable.fromDialog() will return a Maybe<T> rather than an Observable<T> now. This makes sense since a Dialog may only have one response from the user, if there is any provided value at all.

Dialog<String> dlg = ...;

Maybe<String> response = JavaFxObservable.fromDialog(dlg);