qml: fix dragging multiple items in KeyNavigableTableView

This already works fine in other places, I must have missed
updating this in the last iteration.

Brief explanation of how this works:

- Tap handler should set the focus, even if the drag handler
  takes over. This means that the focus should be set in
  both `canceled()` and `singleTapped()` handlers. This
  merely corresponds to `pressed()` signal of mouse area.
- Tap handler should not update the selection if the drag
  handler takes over. This means that the selection should
  not be updated upon `canceled()` but `singleTapped()`
- Drag handler must update the selection when it becomes
  active, as long as the dragged item is not already
  selected.