Separate default controller

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | **no**
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | https://github.com/symfony-cmf/symfony-cmf-docs/pull/304

alternative to #191 without a BC break and a tad more flexibility for edge cases (at the cost of a bit more complexity). by default, everything will work the same as with #191 but for example the SimpleCmsBundle does not fail with this PR whereas #191 would require adjustments there.