Config refactoring

[drew2a]

This PR contains Tribler's config refactoring (mostly renaming).

Also, there is the simplification that @kozlovsky requested:

    def get_path_as_absolute(self, property_name: str, state_dir: Path = None) -> Optional[Path]:
        """ Get path as absolute. If stored value already in absolute form, then it will be returned in "as is".
           `state_dir / path` will be returned otherwise.
        """
        value = self.__getattribute__(property_name)
        if value is None:
            return None
        return state_dir / value