Description
Currently, when you do:

$stopwatch->start('foo');
$event = $stopwatch->stop('foo');
echo (string) $event;

... you're not getting the event's name "foo" in the output:

default: 16.00 MiB - 0 ms

"default" is the category of the event, and since we didn't provide one, it defaults to "default". This is a little weird.

So I'm suggesting:

1. Add the $name property to StopwatchEvent and display the (string) output in the form "category: name", with category defaulting to ''. So in our example the output would be:

   foo: 16.00 MiB - 0 ms

OR

2. Rename $category to $label (to signify that this is not meant to "categorize" anything, but merely the string you will see again in the output), and $name to $id (to signify that this is just an internal ID you won't see again)