plugins/overview: Don't rely on item type to determine drop behavior

Item type is incredibly fragile as a way to determine what was dropped,
as proven by recent breakage. Instead, we can use `Drag.keys` to provide
metadata about what is being dragged/dropped.

src/plugins/overview/qml/main.qml

@@ -541,7 +541,7 @@ FocusScope {
                     }
                     onDropped: (drop) => {
                         drop.accepted = true;
-                        if (drag.source instanceof Item) {
+                        if (drop.keys.includes("kwin-desktop")) {
                             // dragging a desktop as a whole
                             if (drag.source === mainBackground) {
                                 drop.action = Qt.IgnoreAction;
@@ -607,6 +607,7 @@ FocusScope {
                     Drag.supportedActions: Qt.MoveAction
                     Drag.source: mainBackground
                     Drag.hotSpot: Qt.point(width * 0.5, height * 0.5)
+                    Drag.keys: ["kwin-desktop"]
 
                     layout.mode: effect.layout
                     focus: current

src/plugins/private/qml/WindowHeapDelegate.qml

@@ -103,6 +103,7 @@ Item {
         Drag.hotSpot: Qt.point(
             thumb.activeDragHandler.centroid.pressPosition.x * thumb.targetScale,
             thumb.activeDragHandler.centroid.pressPosition.y * thumb.targetScale)
+        Drag.keys: ["kwin-window"]
 
         onXChanged: effect.checkItemDraggedOutOfScreen(thumbSource)
         onYChanged: effect.checkItemDraggedOutOfScreen(thumbSource)