Move the model classes from QtQuick to QtQml

This is needed for proper support of non-GUI instantiators in QtQml.
Only private C++ classes are affected. Aside from name changes, model
classes now operate on QObjects instead of QQuickItems, leading to minor
changes in the implementation of QtQuick classes using them.

The old QML type names will still be registered in the QtQuick import
for the forseeable future, but pointing to the new classes. The new QML
types will be added in a second commit.

Classes Affected:
QQuickVisualDataGroup -> QQmlDataGroup
QQuickVisualDataModel -> QQmlDelegateModel
QQuickVisualItemModel -> QQmlObjectModel
QQuickVisualModel -> QQmlInstanceModel
QQuickChangeSet -> QQmlChangeSet
QQuickListAccessor -> QQmlListAccessor
QQuickListCompositor -> QQmlListCompositor
QQuickPackage -> QQuickPackage (just moved for now)

Change-Id: Ia19e630e53bfa9e5d459e289596cd11df1ea3930
Reviewed-by: Andrew den Exter <andrew.den.exter@qinetic.com.au>