Merge tag '1.5' into develop

Version 1.5
Merge branch 'master' into develop
Require contao/core >=2.11,<4-dev now.
Change tabs to spaces.
drop suggestion of contao-community-alliance/dependency-container-inspector.
Activate ccabs.
Fix code style issues.
Reduce cyclomatic complexity.
Add provisioning of base Contao singletons.
Bump version.
Merge branch 'release/1.6.0-alpha1'