C++ name mappings doesn't consider shared types

[ahayzen]

Note that if this was fixed one can nearly use Q_ENUM_NS to register CXX shared enum as a QML enum, however QML appears to be confused by the enum class T : ::std::intN_t (?). #34

#[cxx_qt::bridge]
mod ffi {
   #[namespace = "Enums"]
   enum MyEnum {
     Thing,
   }

   #[namespace = "Structs"]
   struct MyStruct;
}

If MyEnum or MyStruct are used in properties, invokables, signals, the generated C++ code does not write ::Enums::MyEnum but MyEnum.

This needs the same logic that has been done with namespace attributes on extern blocks.

This can be implemented with #390 where we move from having a combined map to having a cxx_name map and a namespace_map. (consider making a ParserMapping struct with maps inside).