applets/taskmanager: fix move to current desktop for grouped tasks
I tried these scenarios:
- Move single task to current desktop via context menu and middle click.
- Move a group of tasks to current desktop via context menu and middle click.
- Move a single task from a group to current desktop via context menu inside of the group dialog.
Each task in a group is moved to its screen's current desktop. IMO this is the most sensible default, because the "Move to Desktop" functionality doesn't change change the task's screen (nor the screen's desktop). An alternative would be to move all the tasks to the current desktop of the task manager's screen, but that seems a bit weird and it can be achieved by simply using "Move to Desktop N".