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