Swapping desktops: only swap windows on current activity.

In the desktop grid effect, dragging an empty area of a desktop to
another desktop swaps all windows on the two desktops. This change makes
sure that this matches only windows in the current activity.

Was earlier fixed in
plasma/kwin@1e0b0c88
, but the bug was reintroduced by the QML rewrite.

BUG: 386769