Implement better handling of QML property groups

Currently, qdoc does not support a \qmlpropertygroup command, so the handling of grouped QML properties (e.g. anchors in Item) is ad hoc.

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