extend battery names to be more fine-grained

## Problem

We have 16px of width in symbolic icons and 24px and 38px vertically in color icons to work with for battery levels, but we're currently only using 4 levels. We could be providing much more fine-grained battery levels if we wanted

## Proposal

If we stretched the 48px battery icons a couple pixels, we'd have numbers divisible by 8 instead. We could be twice as accurate in showing battery levels

## Prior Art

The freedesktop spec only includes `battery-low` (defined as 20%) and `battery-caution` (defined as 40%)—Which are apparently being used incorrectly right now. The other icon names we're currently using `good`, `full`, and `empty` are just convention from GNOME.