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/node/output/track/track.cpp

@@ -331,7 +331,7 @@ void TrackOutput::ReplaceBlock(Block *old, Block *replace)
   }
 }
 
-TrackOutput *TrackOutput::TrackFromBlock(Block *block)
+TrackOutput *TrackOutput::TrackFromBlock(const Block *block)
 {
   NodeOutput* output = block->output();