return better images in metadata in gstreamer

cover art image uses "preview image". But "image" is better for
that. "preview image" will be used for thumbnail image.

Change-Id: Ic01f878f146b0369eb84e6b153fa68fbc6c54e57
Reviewed-by: Yoann Lopes <yoann.lopes@qt.io>

src/plugins/gstreamer/mediaplayer/qgstreamermetadataprovider.cpp

@@ -93,7 +93,8 @@ static const QGstreamerMetaDataKeyLookup *qt_gstreamerMetaDataKeys()
 
         //metadataKeys->insert(0, QMediaMetaData::CoverArtUrlSmall);
         //metadataKeys->insert(0, QMediaMetaData::CoverArtUrlLarge);
-        metadataKeys->insert(GST_TAG_PREVIEW_IMAGE, QMediaMetaData::CoverArtImage);
+        metadataKeys->insert(GST_TAG_PREVIEW_IMAGE, QMediaMetaData::ThumbnailImage);
+        metadataKeys->insert(GST_TAG_IMAGE, QMediaMetaData::CoverArtImage);
 
         // Image/Video
         metadataKeys->insert("resolution", QMediaMetaData::Resolution);