Update libhandy to 1.8.0

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Version 1.8.0
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- Translation updates
  - Basque
  - Croatian
  - Korean

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Version 1.7.90
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- Remove unnecessary cast in HdyPreferencesGroup
- Translation updates
  - Georgian
  - German

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Version 1.7.0
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- HdyDeck
  - Remove a non-existent property mention
- HdyStyleManager
  - Correctly reset the theme if we're not overriding it.
    Fixes switching away from HighContrast.
  - Fix a typo in docs
  - Remove a leftover debug message
  - Add environment variables to control styles from Builder
- Fix license in docs
- Fix a Meson error when using as a subproject
- Translation updates:
  - Chinese (China)
  - Dutch
  - Indonesian
  - Persian
  - Turkish

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Version 1.6.0
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- Fix introspection-related build warnings
- AdwStyleManager
  - Make color-scheme work with GTK themes ending in -dark
- Translation updates:
  - Basque
  - Hungarian

(prlw1)