Fixed Session load recursion, allow higher level of imbrication

Recursive load of SessionFile in a Session is now detected by filename and prevented. Deeper level of integration of sessionfile inside session is thus possible (set to 3). Sessions now have an id, allowing to reference them by id in the logs. Terminology is clarified between level and recursion.

SessionSource.h

@@ -43,7 +43,7 @@ public:
     void render() override;
 
     // SessionFile Source specific interface
-    void load(const std::string &p = "", uint recursion = 0);
+    void load(const std::string &p = "", uint level = 0);
 
     inline std::string path() const { return path_; }