Mimic declarative QML registration in Qt for Python

In Qt 5.15, we'll have a way to register QML types declaratives, using the QML_ELEMENT macro and its brethren (see commit cc1a604c704f848927b3fa0a97b0a50b0b79d2a4 in declarative / change-id I7abb8a5c39f5e63ad1a0cb41a783f2c91909491b).

It would be nice if PySide could do the same. This would probably entail the following:

• Adding class decorators corresponding to QML(_*)_ELEMENT, which do some reflection and call qmlRegisterType for the user
• A tool which walks the Python AST, and generates a qmltype file for all declaratively registered classes