The main idea is the move of all controller-related code from Application to
ControllerCollection. Reusable applications are now instances of
ControllerCollection instead of Application instances. That way, there is only
one "true" Application instance, and thus only one Pimple container.

Reusable applications can now be packaged as classes, like Extensions. It
allows to package services and controllers into one extension.

Benefits:

 * less hackish (proper wrapping in a class, no need to add a temporary route, no need for the LazyApplication anymore)
 * less code
 * simple and straightforward code (no magic anymore)
 * less side-effects
 * fix most severe/annoying limitations of the current implementation (from what I've read in the mailing-list and the Github issues)
 * better encapsulation of "reusable" applications
 * better separation of concerns
 * simple usage is exactly the same as before (as Application proxies the ControllerCollection methods for the "default" app)

Upgrading is as simple as replacing Application with ControllerCollection for
reusable apps (note that a reusable app is not "standalone" anymore):

    $mounted = new ControllerCollection();
    $mounted->get('/bar', function() { return 'foo'; });

    $app->mount('/foo', $mounted);

A better way now is to create a class:

    use Silex\ApplicationExtensionInterface;

    class FooApplication implements ApplicationExtensionInterface
    {
        public function connect(Application $app)
        {
            $controllers = new ControllerCollection();
            $controllers->get('/bar', function() { return 'foo'; });

            return $controllers;
        }
    }

    $app->mount('/foo', new FooApplication());

Note that you get the "master" application, so that you have access to the
services defined here.

If you want to register services, you can do so in the same extension:

    use Silex\ApplicationExtensionInterface;
    use Silex\ExtensionInterface;

    class FooApplication implements ApplicationExtensionInterface, ExtensionInterface
    {
        public function register(Application $app)
        {
            $app['some_service'] = ...;
        }

        public function connect(Application $app)
        {
            $controllers = new ControllerCollection();
            $controllers->get('/bar', function(Application $app) {
                $app['some_service']->...;

                return 'foo';
            });

            return $controllers;
        }
    }

    $ext = new FooApplication();
    $app->register($ext);
    $app->mount('/foo', $ext);