Add support for filtering VisualDataModels.

Add support for filtering VisualDataModels.

Add a VisualDataGroup element which items within a VisualDataModel
can be assigned to. Setting the group property of a VisualDataModel
or one of its parts models will filter the items visible within a
view to just items belonging to that group.  By default all items
belong to an 'items' group.

The VisualDataModel attached object includes properties indicating
whether a item is a member of a group and its index in the group.

Task-number: QTBUG-21513 QTBUG-21515
Change-Id: If3df6a359a888a6f79923775d2f78076d5e7d2cf
Reviewed-on: http://codereview.qt-project.org/4115
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Martin Jones <martin.jones@nokia.com>