Add "move to activity" entry in taskmanager context menu

Currently (Plasma 5.20.3) moving a window to an activity is a bit convoluted because you have to go through the task manager context menu multiple times. First time to check the activity you want to move the window to and then one or multiples times to uncheck the other activities.

Since this is something I often have to do I decided to propose a fix for that. It's my first contribution to any KDE project so please tell me if I'm doing it wrong.

The feature is working but I'm not a UI/UX guy so feedback on that point is very welcome.

Currently you get a menu like this:

And clicking on one Move to [an_activity] entry makes the window appear only on [an_activity].

EDIT:

Now, only the appropriate activities are listed in the menu. So if a window is on a single activity, there won't be a "move to" entry for that activity.