Add a get function to VisualDataGroup.

Add a get function to VisualDataGroup.

This returns an object with properties for accessing the model data of
an item, membership in the visual data model's groups and the index
of the item in each group.

Task-number: QTBUG-21512

Change-Id: I878442c8a2e30cc6f9941f7412303581613a6142
Reviewed-on: http://codereview.qt-project.org/5515
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Martin Jones <martin.jones@nokia.com>