Refactoring of the services in the backend

[christianlupus]

Is your feature request related to a problem? Please describe.
The idea behind a service is a single topic. The current services do not stick to this pattern. In fact, when #298 should be implemented, clearly separated and small classes are needed to make the test useful.

Describe the solution you'd like
A complete refactory of the services and rework of current PHP file structure would be involved. First, logical parts should be identified and these should then be implemented piece by piece.

I started to work on some of these refactorings due to my work in PR #297. However I did not want to blow up that PR too much and also not blow up any currently ongoing development.
A first step might be to define a rough time schedule to synchronize any currently ongoing development.

To make dependency injection work smoothly, it would maybe be a good idea to have "a single service to rule them all".