batterymonitor: Improve for desktop systems
- As it was so far, it looked like the system was malfunctioning because we were showing a battery with a big fat red mark. If there's no battery we can show the default icon.
- As for the name, we only say "battery and brightness" when there's both of them. Otherwise we show either title.
- If neither batteries or brightness is available, gracefully hide the applet.
BUG: 415073