Make prefix for user repositories directory configurable

Make the currently fixed prefix for the user repositories directory ('~') configurable. Refactor code to gather all uses of the prefix into a new class (for lack of a better name called 'ModelUtils').
An empty prefix is not allowed. The prefix can also be a path relative to the repositories folder. An absolute path is made relative by removing the first '/'. Currently requires reload. I'm not sure if it makes sense to make this dynamically updatable. It should be possible, though.