Creating a general add-on implementation

[MuellerMH]

Add-ons should be able to be added to the project in a simple form. The aim should be for third party developers to be able to write add-ons and use or make them available with TK8. Ideally, these add-ons can be installed and used simply by specifying a git repository.

It would also be possible to provide the add-on, which the later RestAPI can extend with its own endpoints. This should be taken into account in the conceptual design.