Include import name + major import version number in \keyword command for QML types

The lookup for QML type descriptions in Qt Creator is so far done only on base of the QML type name. That is, both the documentation for Qt Quick Controls 1 and Qt Quick Controls 2 have an item Button , which is indexed by the keyword

\keyword QML.Button

This makes it hard to look up the right 'Button'. Instead, qdoc should generate also a keyword that includes the import + major number, e.g.

\keyword QML.QtQuickControls2.Button