[4.4] Use kernel.project_dir and require Symfony 3.3

[Toflar]

http://symfony.com/blog/new-in-symfony-3-3-a-simpler-way-to-get-the-project-root-directory?utm_source=feedburner&utm_medium=twitter&utm_campaign=Feed%3A+symfony%2Fblog+%28Symfony+Blog%29

[leofeyer]

+1 for using kernel.project_dir.

@contao/developers Do we want to keep our contao.root_dir parameter or remove it in favor of kernel.project_dir? Theoretically, contao.root_dir allows us to place the Contao folders into a completely different folder – but who will ever do this?

[ausi]

I think we should get rid of contao.root_dir and use the kernel.project_dir instead.

[aschempp]

We should definitely not add contao.root_dir (which should only have been added in 4.4).

[leofeyer]

See #803.