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.

Session.cpp

@@ -330,6 +330,9 @@ Source *Session::popSource()
         s = *its;
         // remove Node from the rendering scene
         render_.scene.ws()->detach( s->group(View::RENDERING) );
+        // inform group
+        if (s->mixingGroup() != nullptr)
+            s->mixingGroup()->detach(s);
         // erase the source from the update list & get next element
         sources_.erase(its);
     }