Add session bus support for GUI tests, update environment handling:

- Introduce `GUI_EXTRA_ENV` to pass environment variables to the GUI application.
- Add functions for managing a private session bus (`gui_start_session_bus`, `gui_stop_session_bus`) and integrate them into test teardown for cleanup.
- Include utilities for D-Bus tray icon testing (`sni_icon_count`, `sni_wait_icons`, etc.).
- Ensure test case `90_gui.sh` properly resets and handles log ownership to prevent issues with skipped tests.