More fine-grained structure change events for data view

From end-users of the prototype code, there are issues when performing actions like drag-and-drop that add a small number of rows. In these situations the complete refresh of the structure means that the open rows all collapse losing the context of the change and disrupting the user's flow.

Similar issues will occur if we add functionality that permits users to delete selected rows.

We need to be able to signal:

- rows have been added to this parent index
- rows have been deleted from this parent index

It is unclear if we need to specify more (such as the indices locating the additions/removals).