[FrameworkBundle] [Templating] added Stopwatch support to the PHP engine
merged branch bgarret/timed-php-engine (PR #6836)

This PR was merged into the master branch.

Discussion
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[2.3] [FrameworkBundle] [Templating] added Stopwatch support to the PHP engine

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

I did not include tests nor documentation because there weren't any for TimedTwigEngine (I took it as an example). If I'm mistaken and they are needed, I'll gladly write them.

Commits
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3c3d34d [FrameworkBundle] [Templating] added Stopwatch support to the PHP engine
[FrameworkBundle] added a missing entry in the CHANGELOG