applets/taskmanager: reset `parentTask` after a task was moved

After a task item was moved, the property bindings in openWindowToolTipDelegate
also need updated, but to fix bug 467709, `mainItem.parentTask === task`
was added, which causes empty tooltips after a task item was moved.

Test plan:

1. Disable task tooltips and enable Click to open small tooltips
2. Open some Konsole windows to create a task group
3. Open Firefox or any other app
4. Click the Konsole group to activate the tooltip
5. Move Konsole group's position
6. Click the Konsole group again. Before this change the tooltip is
   empty. After this change the tooltip has contents.

Amends 47edbf51

CCBUG: 467709
CCBUG: 452187
FIXED-IN: 5.27.5