Deep redesign of frame grabbers to prevent multiple frame captures

(slow) and optimize distribution of gabbed frames to multiple recording
and streaming pipelines (e.g. record H264 *and* share stream on the
network). New implementation of Loopback source for LINUX using
v4l2loopback (experimental).

Mixer.cpp

@@ -815,9 +815,6 @@ void Mixer::swap()
     session_ = back_session_;
     back_session_ = tmp;
 
-    // swap recorders
-    back_session_->transferFrameGrabber(session_);
-
     // attach new session's nodes to views
     for (auto source_iter = session_->begin(); source_iter != session_->end(); source_iter++)
         attach(*source_iter);
@@ -850,9 +847,6 @@ void Mixer::swap()
     // reset History manager
     Action::manager().clear();
 
-    // inform streaming manager
-    Streaming::manager().setSession(session_);
-
     // notification
     Log::Notify("Session %s loaded. %d source(s) created.", session_->filename().c_str(), session_->numSource());
 }