[WebProfilerBundle] Show AJAX requests in the symfony profiler toolbar

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Adds AJAX requests in the web debug toolbar.
See #8896 for the original discussion.

![image](https://cloud.githubusercontent.com/assets/47313/4384087/43d1feb2-43b0-11e4-99c9-3e50e19e623f.png)

this will trigger lots of reflows, as you are querying the DOM between each update. You should rather get an array with all children and iterate over it instead. This way, there will be no DOM read between the ``removeChild`` calls, meaning that all removals and the appending of the new rows will all happen in a single reflow (and to improve it even more, the DOM query for ``infoSpans`` could be done before these lines, so that updating its text content is part of the same reflow as well)