[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)

I think we should make it private (others should already be private btw as there is no reason to provide extension points here, but we cannot change it for BC reasons)