Added ability to show song cover in HistoryView and Overview::Layout.

Telegram/SourceFiles/data/data_document.cpp

@@ -1488,6 +1488,10 @@ bool DocumentData::isSong() const {
 	return (type == SongDocument);
 }
 
+bool DocumentData::isSongWithCover() const {
+	return isSong() && hasThumbnail();
+}
+
 bool DocumentData::isAudioFile() const {
 	if (isVoiceMessage()) {
 		return false;