Battery Monitor revamp

[Kai Uwe Broulik]

> So I'm a +1 for this change. It will need to be kept backwards compat for
> other themes in the code, but that's not hard to do.

if (fillElementSvg.hasElement("Fill10")) {
  // use new more accurate method
} else {
  // fallback for other/older themes
}

would be the approach for this, right?

> Then we can ditch the text overlay as well, yes? :)
Exactly :-) Let's drop that config dialog!

Will do some Inkscape work sometime tomorrow I guess.