[VarDumper] fix control chars styling
[EventDispatcher] perf optim
[HttpKernel] fix DebugHandlersListener
bug #12285 Various fixes (nicolas-grekas)

This PR was merged into the 2.6-dev branch.

Discussion
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Various fixes

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12278
| License       | MIT
| Doc PR        | -

A 3 in one PR (diff is best viewed [with whitespaces ignored](https://github.com/symfony/symfony/pull/12285/files?w=1)):
- changed the way the DebugHandlerListeners desactivates itself
- reduced a N×N iteration to a N one in ContainerAwareEventListener::removeListener
- fixed an issue in VarDumper

Commits
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ac8efd9 [HttpKernel] fix DebugHandlersListener
5af0f89 [EventDispatcher] perf optim
c7cf6cf [VarDumper] fix control chars styling