qdoc: Implement better handling of QML property groups

The \qmlpropertygroup command is added, and qdoc is taught to generate
better output for it. The format is, e.g.:

\qmlpropertygroup QtQuick2::Item::anchors
\qmlproperty AnchorLine QtQuick2::Item::anchors.top
\qmlproperty AnchorLine QtQuick2::Item::anchors.bottom
\qmlproperty AnchorLine QtQuick2::Item::anchors.left
\qmlproperty AnchorLine QtQuick2::Item::anchors.right
\qmlproperty AnchorLine QtQuick2::Item::anchors.horizontalCenter
\qmlproperty AnchorLine QtQuick2::Item::anchors.verticalCenter
\qmlproperty AnchorLine QtQuick2::Item::anchors.baseline
\qmlproperty Item QtQuick2::Item::anchors.fill
\qmlproperty Item QtQuick2::Item::anchors.centerIn
\qmlproperty real QtQuick2::Item::anchors.margins
\qmlproperty real QtQuick2::Item::anchors.topMargin
\qmlproperty real QtQuick2::Item::anchors.bottomMargin
\qmlproperty real QtQuick2::Item::anchors.leftMargin
\qmlproperty real QtQuick2::Item::anchors.rightMargin
\qmlproperty real QtQuick2::Item::anchors.horizontalCenterOffset
\qmlproperty real QtQuick2::Item::anchors.verticalCenterOffset
\qmlproperty real QtQuick2::Item::anchors.baselineOffset
\qmlproperty bool QtQuick2::Item::anchors.alignWhenCentered

Task-number: QTBUG-32341
Change-Id: I4b06a3a061b23680e663e8d4e82ac9863ffd4ecb
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>