Union: rebase them on Boxed type, add rw access to fields and struct -> Union casting

Some big refactory on Boxed type to use templates in order to make possible to use it as a base for other types (ideally I'd like in future to create other types of boxed types so that we can reduce the number of runtime checks and class/instance members).

As per this, it's possible to redefine union's as boxed types, getting advantage of their features such as:

• Read and write fields
• Construction via object properties

As per this, I've fixed couple of bugs:

• we had an issue if a struct / union had a field named the same way of a method, so now if a field has a .foo field and a .foo() method, the method will have priority, while the field will be accessible via ._field.
• We always tried to use the object constructor if provided, while we can try to use the property-object constructor in case the constructor does not take an object as argument

Finally, added a UnionType.fromMember() static method to make possible to initialize a directly-allocable Union from another Boxed type (struct or union) that is the unique field of such type for an Union... So basically create a Gdk.Event [or Clutter.Event] from a Gdk.EventKey [or Clutter.KeyEvent] (giving access to the union methods and to functions taking the union type).

Fixes: #273

Some tests requires gobject-introspection!348 (merged)