timeline/various: implemented block copy/paste

Implemented the ability to copy/paste blocks/clips in the timeline. This did
require some large scale changes and reworking of the copy/paste system
introduced a few commits ago, but should be largely functional now.

app/render/backend/audiorenderbackend.cpp

@@ -16,12 +16,12 @@ AudioRenderBackend::AudioRenderBackend(QObject *parent) :
 
 void AudioRenderBackend::SetParameters(const AudioRenderingParams &params)
 {
+  CancelQueue();
+
   // Set new parameters
   params_ = params;
 
   // Set params on all processors
-  // FIXME: Undefined behavior if the processors are currently working, this may need to be delayed like the
-  //        recompile signal
   foreach (RenderWorker* worker, processors_) {
     static_cast<AudioRenderWorker*>(worker)->SetParameters(params_);
   }

app/render/backend/videorenderbackend.cpp

@@ -85,10 +85,7 @@ void VideoRenderBackend::SetParameters(const VideoRenderingParams& params)
 
 void VideoRenderBackend::SetOperatingMode(const VideoRenderWorker::OperatingMode &mode)
 {
-  if (!AllProcessorsAreAvailable()) {
-    qCritical() << "Attempted to set operating mode on a backend whose workers are still busy";
-    return;
-  }
+  CancelQueue();
 
   operating_mode_ = mode;