Log Message:
xfce4-tumbler: update to 4.18.2

4.18.2 (2023-11-30)
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- Disambiguate thumbnailer signals
- scheduler: Fix ready/error signal forwarding
- desktop-thumbnailer: Use empty path to build cmdline if it is null
- Update spec link in README
- xdg-cache: Fix temporary file management when saving thumbnail
- Add icon at 64px, clean up SVG metadata