Helper function adding a callback ID to the IDs dict.
    The callback ids dict maps an object to event callback ids.

    :param ids: dict of callback IDs to update
    :param obj: one of the keys of REGISTER_FUNCTIONS
    :param callback_id: the result of _register_callback

def _append_callback_id(ids, obj, callback_id):
    '''
    Helper function adding a callback ID to the IDs dict.
    The callback ids dict maps an object to event callback ids.

    :param ids: dict of callback IDs to update
    :param obj: one of the keys of REGISTER_FUNCTIONS
    :param callback_id: the result of _register_callback
    '''
    if obj not in ids:
        ids[obj] = []
    ids[obj].append(callback_id)