Support TREAT in entity views

[beikov]

As part of our work on TREAT for #120 we should also add support for treat in entity views. There are actually two features that make sense when adding treat support.

1. Allow to treat the "current" entity view
   This requires some syntax to refer to the "current" entity view type.

2. Allow to map an entity view or subview to specific entity view subtypes
   When mapping a subview or applying an entity view it should be possible to specify entity view subtypes that should be materialized when an entity is of the respective entity subtype.

[beikov]

Actually I'd like to allow to map subview subtypes not only based on entity hierarchy but maybe also based on some kind of other discriminator.
Just saw some code that based on the value of a type property produced different values for a generated property based on different property paths i.e.

public String resolve() {
  switch (type) {
    case TYPE1: return getRelation1().getOwner().getName();
    case TYPE2: return getRelation2().getOwner().getName();
    // ...
  }
}