QTableView: Make it possible to indicate where a selection of cells will be dropped precisely rather than just showing the top level cell

QTableView: Make it possible to indicate where a selection of cells will be dropped precisely rather than just showing the top level cell. This would make it clearer to see what would happen when a drop occurs.

Generally speaking, making it easier to see on all platforms where a drop will occur would be good as on Mac especially its not so clear all the time.