support skipping of assistents

[azplanlos]

I want to support skipping of assistants because not all my assistants work linear, also a property is provided for easier access to the complete session dictionary (I use it when I receive the finished notification in my app delegate class). Also resized the special accessory view in the mini version of the TESetupAssistant to window's bottom for easier position calculations (for example to position an extra cancel button in one line with the other default buttons by just copying their y position)