proposed fix for issue #68

Fixes #68 (pardon the crappy VS that changed tons of project files only because I added a solution folder... :-|)
We still have one issue, expected and by design I'd say. The loaded event is still fired more than once by the WPF framework each time a Tab in the TabControl is activated and brought to front.
In this case we can think about caching in the behavior that we already fired the event at least once, I do not really know if it is a good choice or not.