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).

Move window between activities

1. Setup desktop with two activities, taskbar, window on one activity, activate activity with the window on it
2. Open Activity Switcher, Drag window's taskbar entry to the other activity's item.
3. Observe window moves to other activity

Add window to another activity

1. Setup remaining from previous test
2. Open Activity Switcher, Ctrl-drag window's taskbar entry to the other activity's item.
3. Observe window is now on all activities

Move window from all activities to a single activity

1. Setup remaining from previous test. Window is on all activities
2. Open Activity Switcher, Drag window's taskbar entry to an activity item
3. Observe window is now only on one activity

Add window to two out of three activities

1. Setup remaining from previous test. Window is on 1/2 activities
2. Open Activity Switcher, Add 3rd activity using Activity Manager
3. Ctrl-drag window's taskbar entry to the new activity.
4. Observe window is now on 2/3 activities

Move window to a different 2/3 activities

1. Setup remaining from previous test. Window is on 2/3 activities
2. Open Activity Switcher, Activate an activity the window is present on
3. Drag window's taskbar entry to the activity it is not on
4. Observe window is now moved to the activity it was dropped on, and is no longer on the current activity

Add window to the only activity it is not on

1. Setup remaining from previous test. Window is on 2/3 activities
2. Open Activity Switcher, Activate an activity the window is present on
3. Ctrl-drag window's taskbar entry to the activity it is not on
4. Observe window is now present on all activities