Add XmlListModel to qtdeclarative

The initial implementation of XmlListModel is located in qt5/qtxmlpatterns, which would not be a part of Qt6.
 
However it's used in many examples and also mentioned in the user documentation.
 
Some time ago it was already ported to qtdoc repo, so that the examples in the qtdoc could compile. The implementation can be found here: qt5/qtdoc/examples/demos/shared/xmllistmodel.*

However this implementation is internal, and can't be exported to other modules.
We need a more general solution, that could also be used in the user QML code.
 
The idea of this task is to move the updated implementation into qtdeclarative:

• make it a linkable library with installed private header;
• provide an additional optional plugin that contains no code beyond ctor;
• migrate unit tests from xmlpatterns.