Log Message:
libxfce4ui: update to 4.18.3

Change log:

4.18.3 (2023-03-28)
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- Add comment about the need for gtk_widget_destroy()
- Fix memory management in Vala generated code
- Add annotation "(nullable)" where appropriate
- Replace deprecated annotation: "(allow-none)"
- Harden xfce_gtk_handle_tab_accels (thunar#1067)
- screensaver: Avoid Xfconf warning
- shortcuts-grabber: Ignore keyboard layout change
- screensaver: Fix Xfconf memory management