Start converting casting of Managed to subtypes to a better scheme

There is now a template based as<Foo>() method available in
Managed, that'll allow casting to the proper subclass. It uses
the pointer to the vtable and compares it to the classes static
vtable for type checking. Like this we can avoid that Managed has
to know about all subclasses.

Change-Id: Ic966bbe00d85224b8d96cc0ff00f69a6ac67129e
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Move Variant and QtObject over to new casting scheme

Change-Id: Ie9e5e40b4bea4d4379dc9bdd103852c32d6ae478
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>