The xpswatch tray lane measured ~1.9 tray signals/s at idle: refresh()
re-pushed icon (sparkline sample made every pixmap 'new'), tooltip and
menu rows every 2s, each push a NewIcon/PropertiesChanged the shell turns
into StTextureCache growth (the gnome-shell heap). Icon now publishes
only on (temp, gear, trim, badge) change, tooltip on text change with a
10s floor, rows/headers only on change.