OutputHeadV1State: Invalid custom_mode setter

[heuer]

The property setter is invalid. It does the same as the property getter:

    @custom_mode.setter
    def custom_mode(self):
        width = self._ptr.custom_mode.width
        height = self._ptr.custom_mode.height
        refresh = self._ptr.custom_mode.refresh
        return CustomMode(width, height, refresh)

I'll provide a fix together with #149