implemented 'ripple to' timeline actions

app/node/block/block.cpp

@@ -221,3 +221,10 @@ rational Block::MediaToSequenceTime(const rational &media_time)
 
   return media_time - media_in() + in();
 }
+
+void Block::CopyParameters(Block *source, Block *dest)
+{
+  dest->set_block_name(source->block_name());
+  dest->set_length(source->length());
+  dest->set_media_in(source->media_in());
+}

app/node/block/block.h

@@ -102,6 +102,8 @@ protected:
 
   rational MediaToSequenceTime(const rational& media_time);
 
+  static void CopyParameters(Block* source, Block* dest);
+
 private:  
   NodeInput* previous_input_;
   NodeOutput* block_output_;

app/node/block/clip/clip.cpp

@@ -33,8 +33,7 @@ Block *ClipBlock::copy()
 {
   ClipBlock* c = new ClipBlock();
 
-  c->set_block_name(block_name());
-  c->set_length(length());
+  CopyParameters(this, c);
 
   return c;
 }

app/node/block/gap/gap.cpp

@@ -28,8 +28,7 @@ Block *GapBlock::copy()
 {
   GapBlock* c = new GapBlock();
 
-  c->set_block_name(block_name());
-  c->set_length(length());
+  CopyParameters(this, c);
 
   return c;
 }

app/node/output/track/track.cpp

@@ -151,7 +151,7 @@ Block *TrackOutput::NearestBlockBefore(const rational &time)
     }
   }
 
-  return nullptr;
+  return this;
 }
 
 Block *TrackOutput::NearestBlockAfter(const rational &time)
@@ -163,7 +163,7 @@ Block *TrackOutput::NearestBlockAfter(const rational &time)
     }
   }
 
-  return nullptr;
+  return this;
 }
 
 const QVector<Block *> &TrackOutput::Blocks()