Implement selected piece highlighting

Issue #3

frontend/main.js

@@ -31,6 +31,7 @@ class Chess {
     this.canvas = new visuals.Canvas();
     this.configVis = new visuals.ConfigVis(this.backend.getConfig());
     this.configVis.draw(this.canvas);
+    this.activeSquares = new visuals.ActiveSquares();
     this.moveSource = null;
     this.canMoveTo = [];
     document.onclick = (ev) => this.click(ev);
@@ -44,12 +45,14 @@ class Chess {
     if (source === null) return;
     let validMoves = this.backend.getAvailableMoves(source);
     if (validMoves === null) return;
+    this.activeSquares.selectSquare(this.canvas, source);
     this.moveSource = source;
   }
   finalizeMove(target) {
     if (target !== null) {
       this.backend.makeMove(this.moveSource, target);
     }
+    this.activeSquares.unselectSquare(this.canvas);
     this.syncBackend();
     this.moveSource = null;
   }