Make Full Screen Dialogs working with Tabris UI

With issue #118 the problem was raised that working with modal dialogs and Tabris UI does not work properly. This is because the Display size of a Tabris UI application is smaller than the real display.

I think we should consider 2 solutions:

1) Provide an own dialog util to open dialogs full screen. Basically I think this only works around the problem.
2) We should investigate if we can migrate the Tabris UI to use the full display but have a smaller client area to draw things on. I think this would be the best solution because it will not introduce new API.