Be able to use QML_ELEMENT for different namespaces instead of having them all in a single namespace

Be able to use QML_ELEMENT for different namespaces instead of having them all in a single namespace, this makes it easier for complex projects to be able to keep things separate and more reusable for other projects too.

Sometimes, elements should only be accessible by certain parts and thus should only be imported by one section, being able to split these up with the help of the convenience QML_ELEMENT macro and supporting parts would be a very useful to have.