Return application names.

        Return the list of application names that are available in the
        database.

        Returns:
            set of str.

def get_app_names(self):
        """
        Return application names.

        Return the list of application names that are available in the
        database.

        Returns:
            set of str.
        """
        app_names = set()
        for name in self.apps:
            app_names.add(name)

        return app_names