fix activate

slide_model.rs activate function now properly tells of the data that
was changed so that ListViews that use it as a model will reset their
data properly on changes.

The important parts were to ensure we are using the right indexes for
top_left and bottom_right. Then also ensure the roles are correct, but
I think I had that already, just went with the implementation that
CXX-Qt used in their examples.

src/qml/presenter/PreviewSlideListDelegate.qml

@@ -74,11 +74,9 @@ Item {
         target: SlideMod
         function onDataChanged() {
             if (active) {
-                previewHighlight.color = Kirigami.Theme.highlightColor;
                 previewSlidesList.currentIndex = index;
                 previewSlidesList.positionViewAtIndex(index, ListView.Center);
-            } else
-                previewHighlight.color = Kirigami.Theme.backgroundColor;
+            }
         }
     }
 }