Allow shared types as inner Rust types

[LeonMatthesKDAB]

Random idea that occured to me today:

We can get rid of the Box<MyObjectRust> in the C++ QObject, if our MyObjectRust is a shared CXX type.

Our syntax of type MyObject = super::MyObjectRust is already meant to accept other types in the future instead of just super::....

The easiest thing to support is just:
type MyObject = MyObjectRust, where MyObjectRust can be any type that is declared in the bridge.

Now if MyObjectRust isn't an extern type, but a shared type, we could actually omit the Box<> on the C++ side, and have the MyObjectRust inline, which would avoid the overhead of dereferencing.

I know of two challenges with this idea:

1. We must be able to identify whether a given type is shared or opaque.
2. Constructors new function currently returns a Box<CxxQtType::Rust>. Unless we want to break API, we still need to create a box during Construction and then move out of it after. That's not a big deal, but is suboptimal.