CCA code style

Contribution of switching to the cca code styles (https://github.com/MetaModels/core/issues/602).

What this PR contains:
 * Switch of coding standard
 * Split between contao and src folders
 * Some refactoring to pass cyclomatic complexity limit
This is not correct, the "metamodels/php-coding-standard" should be removed (as it got replaced with "contao-community-alliance/coding-standard") and "cyberspectrum/contao-toolbox" should be preserved (as we use it for handling the translations).