Add support for pure virtual methods

[ahayzen-kdab]

We should allow for defining pure virtual methods on RustQt types.

This allows for then creating an abstract QObject and implementing concrete types, for example this is useful when having a real vs mock implementation for testing (this is useful when you have an interface that talks to hardware, over a network socket, to an input device, etc).

In C++ this looks like the following

virtual T ident(...) = 0;

To represent this in Rust we could use #[cxx_pure_virtual] or combing multiple attributes so one is for virtual and one is for the = 0 ?

Once we support this the multi crate example could be a good place to have an example of one crate with an abstract/real/mock implementation and another crate using the abstract version as the type in properties/invokables, as a real world example.

Note that there is also = default in C++, so we need that too ?

[BenFordTytherington]

Is this complete as of #1189?