Smoothly interpolate editor notedelay

This makes editor notes fade out smoothly. And even though the notedelay
only gets decremented by one every editor-frame (the editor runs at
1000/24 FPS fixed-timestep here), it actually gets multiplied by 4, so a
floating-point interpolated value would make a difference here.

desktop_version/src/editor.h

@@ -164,6 +164,7 @@ class editorclass{
 
   int temp;
   int notedelay;
+  int oldnotedelay;
   std::string note;
   std::string keybuffer;
   std::string filename;