video_thumbnail works to be set in rust

Since every slide is asking for their own, we don't need to loop
through all slides, we just use the index to put the thumbnail into
the right slides and emit data changed.

src/cpp/slidemodel.h

@@ -86,6 +86,7 @@ public:
   Q_INVOKABLE QVariantList getItems();
   Q_INVOKABLE int findSlideIdFromServItm(int index);
   Q_INVOKABLE QString thumbnailVideo(QString video, int serviceItemId, int index);
+  Q_INVOKABLE QString thumbnailVideoRust(QString video, int serviceItemId, int index, SlideyMod *slideModel);
   QImage frameToImage(const QString &video, int width);