Log Message:
libxfce4ui: update to 4.18.4

Change log:

4.18.4 (2023-05-22)
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- screensaver: Prevent recursive calling of lock command
- screensaver: Add lock-on-sleep property
- screensaver: Update fallback list
- screensaver: Prioritize user lock command
- screensaver: Rewrite D-Bus proxies management
- screensaver: Use reasonable timeout for methods that may not reply
- screensaver: Remove support for gnome-screensaver (Fixes #62)
- screensaver: Do not take into account empty commands
- screensaver: Simplify Xfconf management
- screensaver: Fix memory leak
- screensaver: Fix typos
- screensaver: Run lock command synchronously (Fixes #70)
- screensaver: Add missing include guard