4.15.5

- Add new API xfce_spawn_no_child (!8):
  Launch processes re-parenting them to init vs. as children
- about: Fix icon names
- about: Display userspace bitness if it differs from OS kernel bitness
- about: Print GPU name in system info
- about: Use more understandable arch labels (Issue #27)
- Improve xfce_icon_name API
- No startup notification in wayland to prevent crash (Issue #17)
- Add missing docstring
- Fix typos
- Add README.md to EXTRA_DIST
- Translation Updates:
  Catalan, Chinese (China), Danish, Galician, German, Italian,
  Lithuanian, Turkish