Accept keys in windowheap conditionally

In WindowHeap we currently always accept the keyboard event regardless
of whether the event was handled.

This will allow the event to propagate to other handlers

src/effects/private/qml/WindowHeap.qml

@@ -315,6 +315,7 @@ FocusScope {
                 }
             }
             if (selectedItem) {
+                handled = true;
                 KWinComponents.Workspace.activeClient = selectedItem.client;
                 heap.activated();
             }