Set up app setting definitions as classes

[mikkonie]

As with many return values of implementable plugin methods, expecting these as dictionaries offers a lot of chance for user error and ambiguity.

We should set up a class for app settings and expect definitions to take form of a list of objects instead of a dict.

This also allows us to place critical validation in the class __init__() to detect badly defined settings as early as possible.

Providing app settings as a dict should be deprecated with warning in v1.1 and removed in v1.2.

Existing setting dicts within SODAR Core should be replaced with the class objects, of course.