use preventStealing in the taskmanager

the taskmanager will take the exclusive in threating all drop events, not colliding with the Panel dnd anymore

BUG:333317

applets/taskmanager/package/contents/ui/MouseHandler.qml

@@ -31,6 +31,7 @@ Item {
     DropArea {
         id: dropHandler
 
+        preventStealing: true;
         anchors.fill: parent
 
         property Item hoveredItem

containments/panel/contents/ui/main.qml

@@ -138,11 +138,7 @@ function checkLastSpacer() {
     }
 
     onDrop: {
-        if (currentLayout.childAt(event.x, event.y) == dndSpacer) {
-            plasmoid.processMimeData(event.mimeData, event.x, event.y);
-        } else {
-            dndSpacer.parent = root;
-        }
+        plasmoid.processMimeData(event.mimeData, event.x, event.y);
     }