Drag, Drop, Copy and Paste support for Data Views

We need the ability for users to drag and copy selections out of the data view; and for users to drop and paste values from elsewhere into the data view.

These are all related as they all involve serialization of data to standard MIME formats (eg. text/plain, text/csv, application/json, etc.) as this is what Qt and Wx use to talk to the underlying OS clipboard, etc.

This will require:

- [x] serialization/deserialzation support for MIME types (#688)
- [ ] hooks to support dragging out values (#704)
- [ ] hooks to allow dropping values into the data view (#706 partially resolves this)
- [ ] cut, copy, paste, etc. methods

Full implementation of this may require the ability to insert/delete on the _structure_ of the data model, rather than simply changing the values of individual items. Some of this is encapsulated by #681 which discusses the events that will need to be generated.