accessibility/atk: update to 2.38.0
Changelog:
- Require Meson 0.56.2
- Add g_autoptr support for ATK types
- Add ATK_STATE_COLLAPSED, matching AT-SPI's state
- Improve support for ATK as a Meson subproject
- Ensure that atk_value_get_value_and_text() can deal with NULL out arguments