Added a new LayersIterator class

This class iterates recursively over layers, flattening groups.
This enables us to simplify the code when we iterate layers. We can remove all recursive function calls in the GameScene code (it is delegated to the LayersIterator)

front/src/Phaser/Map/LayersIterator.ts

@@ -0,0 +1,44 @@
+import {ITiledMap, ITiledMapLayer} from "./ITiledMap";
+
+/**
+ * Iterates over the layers of a map, flattening the grouped layers
+ */
+export class LayersIterator implements IterableIterator<ITiledMapLayer> {
+
+    private layers: ITiledMapLayer[] = [];
+    private pointer: number = 0;
+
+    constructor(private map: ITiledMap) {
+        this.initLayersList(map.layers, '');
+    }
+
+    private initLayersList(layers : ITiledMapLayer[], prefix : string) {
+        for (const layer of layers) {
+            if (layer.type === 'group') {
+                this.initLayersList(layer.layers, prefix + layer.name + '/');
+            } else {
+                const layerWithNewName = { ...layer };
+                layerWithNewName.name = prefix+layerWithNewName.name;
+                this.layers.push(layerWithNewName);
+            }
+        }
+    }
+
+    public next(): IteratorResult<ITiledMapLayer> {
+        if (this.pointer < this.layers.length) {
+            return {
+                done: false,
+                value: this.layers[this.pointer++]
+            }
+        } else {
+            return {
+                done: true,
+                value: null
+            }
+        }
+    }
+
+    [Symbol.iterator](): IterableIterator<ITiledMapLayer> {
+        return new LayersIterator(this.map);
+    }
+}