Adapt Qdoc to use the new property system

QDoc uses the Q_PROPERTY macro to capture the getter, setter, and value change signal info., and append it to the property documentation. With the new system, the clangqdocparser should be able to capture all the details about the QProperty<T> declarations, but the docpaser might have to be taught to use this info. in a sensible way.  

Here are some useful links:

• https://web.microsoftstream.com/video/81ae2b98-f177-4532-ab5f-0216dcf70647 - link to the video recording where the new property system is described.
• Related gerrit changes that could be used as a reference: 
  • https://codereview.qt-project.org/c/qt/qtdeclarative/+/305129
  • https://codereview.qt-project.org/c/qt/qtbase/+/307151
  • https://codereview.qt-project.org/c/qt/qtbase/+/304988/4
  • https://codereview.qt-project.org/c/qt/qtbase/+/307481

The getter will always be value() irrespective of the property type, and the setter is a simple assign statement using the = operator. We could just append this info. to the documentation for every QProperty<T> instance found.

The value change notification signal will always be onValueChanged and the developer must register a callback using the QProperty::subscribe method to get notified:

template <typename Functor> QPropertyChangeHandler<Functor> QProperty::subscribe(Functor f)

For more info., see the documentation for QProperty::subscribe().