Make our event base class independent of Symfony and follow PSR

Symfony is migrating towards a pure PSR event dispatcher, hence their
event class is POPO that implements the PSR stoppable event interface.
Since we can do that ourself and this change doesn't come with any API
changes (breakage), it's easy for us to become independent of Symfony
but also stay PSR-compliant at the same step.

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>