qdoc: \l now works with QML property groups

This update makes it possible to link to the name of a
QML property group with a \l command. e.g. \l {Item::anchors}

Task-number: QTBUG-35844
Change-Id: I2dc51bd90eadd13754a8f4d3357f3f17605a068d
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>