Refactor and introduce tests

[BitTim]

This issue will be active for quite a while as I will occasionally introduce tests for previously untested code. New code from this point forward should be covered by tests.

This issue will also not feature a separate branch as I will introduce these tests while working on other issues.

This will be done to improve the development experience and to improve code quality.

[BitTim]

Since I have expanded fhe scope of this issue to also refactor most of the code base, this issue will get an own branch.

[BitTim]

Refactoring also includes modularization of the code base to make future migrations, like proposed in #50, easier