qdoc: QML "inheritance" parsing

We can use the top-level item in a QML file to follow the chain of QML object inheritance.

In ChildButton.qml:
CommonButton

{ property childprop }

In CommonButton.qml:
SuperParent

{ property commonprop }

In SuperParent.qml:
Item

{ property x,y,z }

So, in the ChildButton docs:
properties:
childprop
commonprop
x, y, z