Expose to/from Qt indices in DataView

Users report that these methods are potentially useful when extending the capabilities of the DataView for Qt: https://github.com/enthought/pyface/blob/a44a5592f6df50101cd7449c4f0bfe4f41a7d7f5/pyface/ui/qt4/data_view/data_view_item_model.py#L346-L366

Other than getting access to the index, these don't particularly make use of the Qt item model, and the logic around Qt indices can be difficult to understand. As a result, having the core logic moved out to utility methods may make some sense and allow code re-use.