Encode layer name in tile factory, remove get_elem()

ts/tile_factory.ts

@@ -1,10 +1,12 @@
 import { Tile } from './tile.js';
 
 export abstract class TileFactory {
+  layer_name: string;
   width: number;
   height: number;
 
-  constructor(width: number, height: number) {
+  constructor(layer_name: string, width: number, height: number) {
+    this.layer_name = layer_name;
     this.width = width;
     this.height = height;
   }