Imageviewer: add button to copy image as data url

Change-Id: Ia30751ebb9b567b22de9d1256bb59aa2116366ba
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/plugins/imageviewer/imageviewer.h

@@ -56,6 +56,7 @@ public:
 
     void exportImage();
     void exportMultiImages();
+    void copyDataUrl();
     void imageSizeUpdated(const QSize &size);
     void scaleFactorUpdate(qreal factor);