Add xfce config set

[RWejlgaard]

Summary

• Adds a new xfce config set (config/sets/xfce/) alongside the existing kde/gnome/macos sets, applied via xfconf-query.
• Same keyboard repeat rate and Caps Lock as Escape as kde/gnome, screen locking and idle display dim/off disabled, device automounting disabled, and an editable path bar in Thunar and GTK file dialogs.
• Sets a dark theme, picking whichever of a few common dark theme packages is actually installed.
• Collapses the default two-panel layout down to a single panel, docked to the bottom of the screen.
• Installs Kitty (per-distro package manager) and sets it as the default terminal via ~/.config/xfce4/helpers.rc.
• Documents the new set in the README, matching the existing kde/macos entries.

Every section is independent, gated on the relevant binary being installed, and safe to re-run.

Test plan

• shellcheck on config/sets/xfce/apply.sh passes clean
• Ran the script end-to-end (twice, to check re-run safety) against a live xfconfd in a sandboxed D-Bus session with a seeded two-panel config; confirmed the second panel and its plugins are removed, the primary panel is repositioned to the bottom, and keyboard repeat settings are written correctly
• Not tested against a real XFCE session (no XFCE machine available in this environment) — worth a sanity check on an actual XFCE box before relying on it