[applets/systemtray] Follow panel opacity

The system tray doesn't follow the panel opacity (it's always
transparent even when the setting is set to opaque or dynamic
with a maximised window present)

BUG: 439025
FIXED-IN: 5.24.0
2 jobs for !1323 with 439025_systray_follow_planel_opacity in 7 minutes and 2 seconds
latest merge request