xfce4-xkb-plugin: update to 0.8.3

Change log:

0.8.3
======
- Add optional notification support (Fixes #48)
- Disable notifications by default
- Use symbolic icon for notification
- Add new plugin icon
- Use Xfce's icon name
- Fix make distcheck
- build: Fix intltool lock file problem during make distcheck
- Fix compilation warnings
- autoconf: Some updates
- Update `.gitignore`
- Translation Updates:
  Albanian, Basque, Belarusian, Bulgarian, Catalan, Chinese (China),
  Chinese (Taiwan), Dutch, English (Canada), Estonian, French,
  Galician, Georgian, German, Hebrew, Italian, Japanese, Lithuanian,
  Norwegian Bokm奪l, Occitan (post 1500), Polish, Portuguese, Russian,
  Serbian, Slovak, Slovenian, Spanish, Swedish, Turkish, Ukrainian

(gutteridge)