Unified & fixed implementation of Group of sources (formerly flatten)

Fixed MixingGroup keep&restore when making Session Group Sources. New global feature to Group all sources into one session source. Unused but potentially useful implementation of flatten of mixer session into one new session source.

Mixer.h

@@ -66,7 +66,10 @@ public:
 
     // operations on selection
     void deleteSelection  ();
-    void flattenSelection ();
+    void groupSelection ();
+    void groupAll ();
+
+    void flattenSession();
 
     // current source
     Source *currentSource ();
@@ -113,7 +116,6 @@ public:
     void setResolution(glm::vec3 res);
     bool busy   () const { return busy_; }
 
-    void flatten();
 
     // operations depending on transition mode
     void close  (bool smooth = false);