- audioplayer: show cover from id3-tag or folder.jpg in header

if exist a file called folder.jpg in same dir as the audiofile
  or a audiocover is defined in id3-tag it will be displayed in header.

  cover in tag is preferred.

  ported from mohusch. original patch by tangocash. thx!

src/driver/audiometadata.h

@@ -84,6 +84,7 @@ public:
 	std::string date;
 	std::string genre;
 	std::string track;
+	std::string cover;
 	bool changed;
 };
 #endif /* __AUDIO_METADATA__ */