[RTM] Release/4.0.0

[dmolineus]

This is a WIP rework of merger² to get it Contao 4.4 ready. I decided to drop Contao 3 support for any further installions.

Roadmap for release:

• [x] Switch to ContaoCommunityAlliance namespace
• [x] Register functions as services
• [x] Use suncat/mobile-detect-bundle instead of our own integration
• [x] Make tests work again
• [x] Apply coding standards

[dmolineus]

I updated the merger² to support Contao 4. Please check if everything is fine. I'm willing to maintain merger² in the furture.