allowed any callback to be a method call on a service (a:b notation)

#800 added the possibility to use a service method (with the `a:b` notation) for converters. This PR goes one step further by allowing almost all methods accepting a callback to use a service method.

The following is now possible:

```php
$app->before('service:beforeApp');
$app->after('service:afterApp');
$app->finish('service:finishApp');
$app->error('service:error');
$app->on('kernel.request', 'service:onRequest');

$app
    ->match('/', 'service:controller')
    ->convert('foo', 'service:convert')
    ->before('service:before')
    ->after('service:after')
;
```