Abstract away or eliminate most of the mutation of the Elements array (#955)

src/scene/createScene.ts

@@ -1,6 +1,17 @@
 import { ExcalidrawElement } from "../element/types";
 
+class SceneState {
+  constructor(private _elements: readonly ExcalidrawElement[] = []) {}
+
+  getAllElements() {
+    return this._elements;
+  }
+
+  replaceAllElements(nextElements: readonly ExcalidrawElement[]) {
+    this._elements = nextElements;
+  }
+}
+
 export const createScene = () => {
-  const elements: readonly ExcalidrawElement[] = [];
-  return { elements };
+  return new SceneState();
 };