libtaskmanager: consolidate group parent adjacent to the moved task

If the task exchanges position with a group parent, the group parent
also needs to be consolidated.

Test:

1. Open many Konsole windows
2. Open Firefox (One window)
3. Now the task order is [Firefox] [Konsole Group Parent]
4. Move Firefox to the right of Konsole group parent.
5. Open group dialog of Konsole group parent
6. Move the first Konsole window down to the second position
7. Before this change, Firefox and Konsole group parent will exchange
   positions, which is incorrect. After this change, the Konsole task in
   the group dialog can be moved as expected.