In the same way that the Activity Pager should (see https://phabricator.kde.org/D27322 for fix) accept dragged windows to move them between Activities, this patch allows the Activity Switcher (meta+q) to move/add (ctrl-drag) windows between/to Activities.
Compare to https://cukic.co/2014/07/15/a-screencast-of-the-activity-switcher-in-plasma-5-1/ (video at https://www.youtube.com/watch?v=uxaDaXW67Oo).