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.

SessionCreator.h

@@ -67,6 +67,7 @@ public:
 protected:
     // result created session
     Session *session_;
+    std::string sessionFilePath_;
     // parsing current xml
     tinyxml2::XMLElement *xmlCurrent_;
     // level of loading recursion