taskmanager: Show "Pin to Task Manager" menu on already pinned launchers

GUI: This makes it more consistent to the menus already shown on running tasks:

I'm not sure if this is intentional design or not, but currently moving pinned tasks around activities is not a good workflow. Before this change, if you say - pinned an app to the wrong activity you would have to launch the application again to pin it to another activity :-(

This only changes the menu shown on already pinned launchers, and doesn't change the fact that you can't pin to the task manager on a specific activity directly from the kicker menu - I'm planning to do that next :-)

I was a little worried about the naming of the menus, however I noticed that it's identical to the menu shown on running tasks so I guess it's not an issue:

By the way this is my first KDE contribution (yay!) so I think my small code change isn't too bad, but I'm happy to accept feedback.