added simple sleep state for elements

refactoring

Scripts/Elements/Dirt.cs

@@ -10,11 +10,4 @@ public class Dirt : Solid {
         DiffuseSpeed = 50;
     }
 
-    public override bool Update(int currentFrame) {
-        if (!base.Update(currentFrame)) return false;
-
-        Tick();
-
-        return true; // not necessarily end, subclasses could do some more things
-    }
 }