added simple sleep state for elements

refactoring

Scripts/Elements/Solid.cs

@@ -5,4 +5,12 @@ namespace FOU.Scripts.Elements;
 public abstract class Solid : Element {
     protected Solid(int x, int y, ref Level level) : base(x, y, level) { }
 
+    public override bool Update(int currentFrame) {
+        if (!base.Update(currentFrame)) return false;
+        if (LastMove + STEPS_UNTIL_INACTIVE < currentFrame) Active = false;
+
+        Tick();
+
+        return true; // not necessarily end, subclasses could do some more things
+    }
 }