[WIP] AppKernel in the bundle

Using this concept, people can use the Contao foundation and add their own bundles and configuration to the `app` directory. The app directory will the be installation-specific and never needs to be updated/overwritten.

This can be powerful for developers as they can now create a full-fledged Symfony application and add the Contao frontend controllers to it (a little bit like NEOS for FLOW).

It also means we do not need to add anything to the `contao/contao` repository whenever we change something on the bundle configuration (easier for development and pull requests).

Needs complementing pull request at contao/contao#20


TODO:
- [ ] Move some bundle dependencies to the bundle's `composer.json`


*PS: the `routing.yml` will be emptied later on with my upcoming "dynamic routing" ;-)*

Up for discussion @contao/developers