Add life cycle method for initialization of ViewModels

[manuel-mauky]

In Views the developer can use the initialize method to setup bindings to the ViewModel and for other initialization logic. We support this for pure java code Views too.

But at the moment there is nothing like this for ViewModels.

When the ViewModel has a method with the signature void initialize() we will invoke this method after all mvvmfx specific injection is done. At the moment we have no such injection features but in the future this can be usefull, for example for the injection of ResourceBundles (#176) and scopes (#154).