Add a button to delete the selection in canvas

www/whiteboard/index.html

@@ -23,6 +23,7 @@
     <div id="controls">
         <button id="clear" data-localization="canvas_clear">Clear</button>
         <button id="toggleDraw" data-localization="canvas_disable"></button>
+        <button id="delete" style="display: none;" data-localization="canvas_delete"></button>
         <input id="width" type="range" value="5" min="1" max="100"></input><label for="width">5</label>
         <span class="selected"></span>
     </div>