implemented value node

Also implements changes necessary to support an input data type
that changes. Much of that foundation was built in
`nodearchchanges`, but hadn't been finalized. This commit
finalizes and provides a reference implementation/test with the
"Value" node.

Fixes #1443

app/node/inputimmediate.h

@@ -151,12 +151,19 @@ public:
     return (!is_keyframing() || keyframe_tracks_.at(track).isEmpty());
   }
 
+  void set_data_type(NodeValue::Type type);
+
 private:
   /**
    * @brief Non-keyframed value
    */
   SplitValue standard_value_;
 
+  /**
+   * @brief Default value
+   */
+  SplitValue default_value_;
+
   /**
    * @brief Internal keyframe array
    *