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/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;