Implement moving on engine

Thus make all state management the engine's responsibility, and remove
the state maintaining code from frontend!

frontend/visuals.js

@@ -160,7 +160,7 @@ export class ConfigVis {
     this.piecesVis = new Map();
   }
   draw(canvas) {
-    for (let [position, piece] of this.configuration.board) {
+    for (let [position, piece] of this.configuration) {
       if (this.piecesVis.has(position)) {
         if (this.piecesVis.get(position).piece == piece) {
           continue;
@@ -175,7 +175,7 @@ export class ConfigVis {
       this.piecesVis.set(position, pv);
     }
     for (let [position, pv] of this.piecesVis) {
-      if (!this.configuration.board.has(position)) {
+      if (!this.configuration.has(position)) {
         pv.undraw();
         this.piecesVis.delete(position);
       }