Added initialization of custom action elements

Hey there,

I'm using your XFormsProcessor for a little framework I'm currently working on. 
When I tried using my own custom elements with betterFORM I came across some null pointer exceptions. I tracked them down to the Initializer class not initializing custom action elements properly, which in return led to null pointer exceptions when the action was triggered.

I added the initialization of custom action elements to the `initializeModelConstructActionElements()` and `initializeActionElements()` methods and also reduced some redundancies through using the already present `initXFormsObject()` method.

I hope you'll find this useful, best regards
Karl