I just added experimental functions `onDock` and `...

# tornadofx

[edvin]

I just added experimental functions onDock and onUndock to UIComponent. They will be called when a new parent or scene is attached or detached from the root node of the View. Will be handy for stuff like initialization (actually better than init in some cases), and cleanup (undock). Please let me know what you think or if you run into problems. It might need some tweaking though.