Implementation of user defined mixing deactivation limit

Mixing view handles to grab and scale limbo area. Saving of user defined limit in Session (and snapshot). Testing for source activation outside of update during session update loop.

SessionVisitor.cpp

@@ -70,6 +70,9 @@ bool SessionVisitor::saveSession(const std::string& filename, Session *session)
         // source visitor
         (*iter)->accept(sv);
 
+    // save session attributes
+    sessionNode->SetAttribute("activationThreshold", session->activationThreshold());
+
     // save the thumbnail
     FrameBufferImage *thumbnail = session->thumbnail();
     if (thumbnail != nullptr && thumbnail->width > 0 && thumbnail->height > 0) {