Returns the values associated with @key under @group_name.
In the event the key cannot be found, %NULL is returned and
@error is set to #G_KEY_FILE_ERROR_KEY_NOT_FOUND. In the
event that the @group_name cannot be found, %NULL is returned
and @error is set to #G_KEY_FILE_ERROR_GROUP_NOT_FOUND.
Returns the values associated with @key under @group_name.
In the event the key cannot be found, %NULL is returned and @error is set to #G_KEY_FILE_ERROR_KEY_NOT_FOUND. In the event that the @group_name cannot be found, %NULL is returned and @error is set to #G_KEY_FILE_ERROR_GROUP_NOT_FOUND.