made battery module much more flexible

So I can have my old representation back... indeed you can
do pretty much anything now with it:

status.register(battery,
    format="{consumption:.2f}W {percentage:.2f}% [{percentage_design:.2f}%] {remaining_hm}"
)
backlight module

Probably I'll discard these as soon as I come up with a nice
generic way of reading multiple files, converting values etc.