qml: fix multiple items can not be dragged in KeyNavigableTableView regression

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.

Regression since !4357 (merged).