Log Message:
xfce4-panel: update to 4.18.4

Change log:

4.18.4 (2023-05-22)
======
- icons: Scale at size 16x16 to 128x128 (Fixes #385)
- icons: Fix centering of org.xfce.panel
- icons: Rename org.xfce.panel.statustray to org.xfce.panel.systray
- Fix memory leaks around gtk_get_current_event()
- tasklist: Fix use-after-free on tasklist child
- systray: Update icon also on status change
- Translation Updates:
  Catalan, Estonian, French, Greek, Italian, Korean, Occitan (post
  1500), Russian, Slovenian, Turkish, Ukrainian