Make it easier to select a custom proxy object with a class name different from the QML type

Since bug #1210260 was fixed, custom proxy objects can now extend the classmethod validate_dbus_object in order to overwrite the QML types that it matches.

However it's still hard to get it right. I keep getting other objects matching the validation, and in order to make it work I can't pass the class as the first parameter of select_single.

This should be easier and clearer. Lets say that I have many QML types 'ItemPage', each one with a different object name. If I want to make a different custom proxy object for each one, I should be able to do:

main_view.select_single(TestPage1, objectName='page1')

main_view.select_single(TestPage2, objectName='page2')

where TestPage1 and TestPage2 are not on the QML tree.