taskmanager: Prevent useless component creation/destruction for grouped tooltips

The binding of "model" property of DelegateModel in ToolTipDelegate.qml
will trigger useless component creation/destruction when "rootIndex" is
changed but is not undefined. Make the ternary operator depend on
"hasRootIndex" bool property to avoid that.

In Task.qml, only assign different values to "parentTask" and "rootIndex"
to avoid unnecessary component creation/destruction for the second time
hovering on the same task.

CCBUG: 433907
CCBUG: 444001


(cherry picked from commit 12373d31)