Add a fps-counter widget to some Sky demos

sky/examples/spinning-square.sky

 #!mojo mojo:sky_viewer
 <sky>
+<import src="fps-counter.sky" />
 <style>
 square {
   margin: 50px;
   height: 100px;
   width: 100px;
   background-color: green;
 }
 </style>
-<square></square>
+<square />
+<fps-counter showHistory="true" />
 <script>
 var square = document.querySelector('square');
 var timeBase = 0;
 
 function animate(time) {
   if (!timeBase)
     timeBase = time;
   var delta = time - timeBase;
   var rotation = Math.floor(delta / 10);
   square.style.transform = 'rotate(' + rotation + 'deg)';
   requestAnimationFrame(animate);
 }
 
 requestAnimationFrame(animate);
 </script>
 </sky>