removed "previous_input" param from block

It was becoming inevitable to get rid of this, it was continuously an exception
that had to be made to the DAG concept seeing as blocks are never really
"dependents" on each other, i.e. the images they produce have nothing to do
with other blocks. The only exception is a transition which will also be
easier to accomplish with this slightly different design.

app/node/block/block.h

@@ -59,8 +59,6 @@ public:
   Block* previous();
   Block* next();
 
-  NodeInput* previous_input();
-
   NodeOutput* buffer_output();
   NodeOutput* block_output();
 
@@ -116,29 +114,22 @@ protected:
   static void CopyParameters(Block* source, Block* dest);
 
 private:
-  NodeInput* previous_input_;
   NodeOutput* block_output_;
-
   NodeOutput* buffer_output_;
 
   rational in_point_;
   rational out_point_;
 
   rational length_;
-
   rational media_in_;
 
+  Block* previous_;
   Block* next_;
 
   QString block_name_;
 
   QVector<Block*> linked_clips_;
 
-private slots:
-  void EdgeAddedSlot(NodeEdgePtr edge);
-
-  void EdgeRemovedSlot(NodeEdgePtr edge);
-
 };
 
 #endif // BLOCK_H