Doc: Add QML Types to table of contents in Assistant

Replace 'headerfile' with 'qmlclass' in qdocconf file.

As a positive side-effect, QDoc will also add files for 'All members'
for QML Types into .qhp as well.

Task-number: QTBUG-29226
Change-Id: I282ef42283ec147ea0f2b466b15e7f104c8a012a
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>

src/quick/doc/qtquick.qdocconf

@@ -20,7 +20,7 @@ qhp.QtQuick.customFilters.Qt.filterAttributes = qtquick 5.0.1
 qhp.QtQuick.subprojects         = qmltypes classes examples
 qhp.QtQuick.subprojects.qmltypes.title = QML Types
 qhp.QtQuick.subprojects.qmltypes.indexTitle = Qt Quick QML Types
-qhp.QtQuick.subprojects.qmltypes.selectors = fake:headerfile
+qhp.QtQuick.subprojects.qmltypes.selectors = fake:qmlclass
 qhp.QtQuick.subprojects.qmltypes.sortPages = true
 qhp.QtQuick.subprojects.classes.title = Classes
 qhp.QtQuick.subprojects.classes.title = C++ Classes