UIComponent: clear stoppedTimers in animationFrame

Otherwise, this Set will grow in size infinitely, potentially causing
performance and memory issues.

GitHub: #133
Linear: EM-2325

src/main/kotlin/gg/essential/elementa/UIComponent.kt

@@ -752,6 +752,7 @@ abstract class UIComponent : Observable(), ReferenceHolder {
         }
 
         stoppedTimers.forEach { activeTimers.remove(it) }
+        stoppedTimers.clear()
     }
 
     open fun alwaysDrawChildren(): Boolean {