This update ensures the panel's tray is hidden when there are no visible items and updatesits visibility automatically when the configuration changes or when notifications are added
or removed.
New Features:
- Add `_update_tray_visibility` method that hides the tray when empty and updates UI
accordingly.
- Connect `tray` "add" and "remove" signals to trigger visibility updates.
- Adjust chevron button visibility, icon, and left margin based on tray state.
Enhancements:
- Invoke visibility update on panel initialization, DND changes, and configuration changes.
- Reset visual elements (chevron icon, margin) when the tray becomes hidden.
- Preserve layout spacing improvements when tray is hidden.
Bug Fixes:
- Prevent stale tray visibility state after configuration updates.