[WIP][WebProfilerBundle] Load web profiler panels asynchronously

Just built this as an experiment to see if having profiler panels lazy-loaded after page load then rendered instantly when they are needed would improve the experience of using the profiler.

I think I uncovered a bug in Twig whilst building this, which prevents this PR from working without tweaking cached templates. Will report on the twig repo.

Also one other problem is the JS/canvas timeline not rendering, but I'm sure it is easily fixed. Edit: due to this: http://stackoverflow.com/questions/1197575/can-scripts-be-inserted-with-innerhtml. Easily fixable