Fixes#1329Fixes#3317 (duplicate)
Examples where the tray "disappeared":
- Start without a "tray_output", reload config with "tray_output none",
remove "tray_output none" and reload.
- Start with "tray_output primary" (or "tray_output <output>"), disable
the primary output and then reconnect it.
Fixes#2010
Cached in output_for_tray