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
1e0b0c881e
, but the bug was reintroduced by the QML rewrite.

BUG: 386769

src/effects/desktopgrid/desktopgrideffect.cpp

@@ -199,7 +199,7 @@ void DesktopGridEffect::swapDesktops(int from, int to)
     QList<EffectWindow *> fromList;
     QList<EffectWindow *> toList;
     for (auto *w : effects->stackingOrder()) {
-        if (!w->isNormalWindow()) {
+        if (!w->isNormalWindow() || !w->isOnCurrentActivity() ) {
             continue;
         }
         if (w->isOnDesktop(from)) {