Include running periods in duration

StopwatchEvent:
  - method getDuration() now includes periods that are not stopped yet

StopwatchEventTest:
  - added testDurationBeforeStop()
Allow retrieving unstopped stopwatch events

Section:
  - added method getEvent()

Stopwatch:
  - added method getEvent()

StopwatchTest:
  - modified testStart() to test for getEvent() method
  - added testUnknownEvent()
feature #10198 [Stopwatch] Allow getting duration of events without calling stop() (jochenvdv)

This PR was merged into the 2.5-dev branch.

Discussion
----------

[Stopwatch] Allow getting duration of events without calling stop()

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | [#10175](https://github.com/symfony/symfony/issues/10175)
| License       | MIT
| Doc PR        | [#3539](https://github.com/symfony/symfony-docs/pull/3539)

Commits
-------

2efe461 Allow retrieving unstopped stopwatch events
d3d097d Include running periods in duration