Demos: Improve page performance by pre-rendering nav menu collapsibles

[gabrielschulhof]

I added a console.log to the widget factory so I can get a tally of what widgets we instantiate in the demos as you load one demo after the other. Our most-instantiated widget? The humble collapsible. That's because it's present in the left-hand-side navigation menu, which in turn is present on, like, all demo pages.

So, I wrote a test that loads (almost) all demo pages and measures how long they take to load. By pre-rendering the collapsibles in demos/jqm-contents.php the total time it takes to sequentially load the pages listed in the test was reduced in length by nearly 4s.