effects/desktopgrid: Fix cursor shape being lost while dragging a desktop

DragHandler does not travel along with its target heap, so it only
affects the cursor shape when the mouse pointer is directly over it; but
when a desktop is dragged far outside, the cursor resets.

Besides, this is how we already implemented it in WindowHeapDelegate.

FIXED-IN: 5.26

src/effects/desktopgrid/qml/DesktopView.qml

@@ -88,7 +88,6 @@ FocusScope {
         id: dragHandler
         target: heap
         grabPermissions: PointerHandler.ApprovesTakeOverByHandlersOfSameType
-        cursorShape: active ? Qt.ClosedHandCursor : Qt.ArrowCursor
         onActiveChanged: {
             if (!active) {
                 heap.Drag.drop();
@@ -162,6 +161,12 @@ FocusScope {
         }
     }
 
+    MouseArea {
+        anchors.fill: heap
+        acceptedButtons: Qt.NoButton
+        cursorShape: dragHandler.active ? Qt.ClosedHandCursor : Qt.ArrowCursor
+    }
+
     PC3.Control {
         id: desktopLabel
         anchors.margins: PlasmaCore.Units.largeSpacing