Implement shake-to-reload

Apps can import this library if they wish to reload on shake.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/963303004

examples/touch-demo.sky

@@ -1,5 +1,6 @@
 #!mojo mojo:sky_viewer
 <sky>
+<import src="/sky/framework/debug/shake-to-reload.sky" />
 <style>
 dot {
     position: absolute;