[Stopwatch] Add getEvents() to Stopwatch

Description
There is getSectionEvents(), but if you didn't open a Section, there is no way to get all Stopwatch events. Sure you can get each single event with $event = $stopwatch->stop('foo'); – but you need to know the events' names. So what if you have an unknown number of events? Right now, I'm opening a (useless) Section, just to be able to do getSectionEvents().