Support for relative path for files in mix

File path in mix session file add a relative reference to the location of the session mix file. If SessionCreator cannot find the absolute path, it tries to load the file at the relative location. Done for MediaSource, SessionFileSource and SequenceSource.

MediaPlayer.cpp

@@ -232,6 +232,9 @@ void MediaPlayer::open (const std::string & filename, const string &uri)
     else
         uri_ = uri;
 
+    if (uri_.empty())
+        failed_ = true;
+
     // close before re-openning
     if (isOpen())
         close();