xfce4-systemload-plugin: update to 1.3.1

Change log:

1.3.1 (2021-03-22)
=====
- Update README.md
- Update and sort the list of authors
- Add a simple network bandwidth monitor (uses libgtop as a fallback)
- Remove 4-valued history
- Fix initial progress bar value
- Fix system-monitor-command setting (Fixes #15)
- Distinguish uptime from the appearance of a digital real-time clock
- Replace "Options" with "Label" in the properties dialog
- Finish porting to xfconf
- Update docs URL
- Code cleanups
- Translation Updates:
  Bulgarian, Chinese (China), Chinese (Taiwan), Hebrew, Italian,
  Japanese, Norwegian Bokm奪l, Polish, Portuguese, Portuguese (Brazil),
  Slovak, Spanish, Swedish

1.3.0 (2021-03-10)
=====
- Port to xfconf
- Add new icons and switch to rDNS naming
- Add Help button to properties dialog
- Fix compilation warnings
- autoconf: Some updates
- Update `.gitignore`
- configure.ac: use AC_PROG_CC_C99
- Translation Updates:
  Belarusian, Bulgarian, English (Canada), Hungarian, Russian, Turkish,
  Ukrainian

1.2.4 (2020/12/21)
------------------
- Fix missing-prototypes
- Remove GSourceFunc casts
- Fix the properties dialog and a hidden memory corruption
- Avoid repainting bars if there are no visual changes
- Update copyright year
- Update URLs
- Create README.md and update configure.ac.in
- Add basic GitLab pipeline
- Update URLs from goodies.x.o to docs.x.o (Bug #16178)
- Allow compilation with panel 4.15
- Translation Updates:
  Albanian, Arabic, Basque, Belarusian, Bulgarian, Catalan, Chinese
  (China), Chinese (Taiwan), Croatian, Czech, Danish, Dutch, Eastern
  Armenian, English (Australia), English (Canada), English (United
  Kingdom), Estonian, Finnish, French, Galician, German, Greek, Hebrew,
  Hungarian, Icelandic, Indonesian, Interlingue, Italian, Japanese,
  Korean, Latvian, Lithuanian, Malay, Norwegian Bokm奪l, Occitan (post
  1500), Polish, Portuguese, Portuguese (Brazil), Russian, Serbian,
  Slovak, Slovenian, Spanish, Swedish, Thai, Turkish, Ukrainian, Urdu,
  Urdu (Pakistan), Uyghur

(gutteridge)