taskmanager: Update rootIndex when visibility is changed
When there are two grouped tasks named A and B, and the position of B is swapped with A (by dragging B), user can still see old items of A in the textual list of grouped tasks B.
This is because in the original binding expression, both visualParent and itemIndex are not changed, so QML will not re-evaluate the expression.
To only update rootIndex when groupDialog.visible is true,
Binding.when is used, and Binding.RestoreNone is to avoid
sudden changes in height.