* frontlight on kobo: a few fixes
Rewritten to not update NickelConf on every change, and never
if KOBO_SYNC_BRIGHTNESS_WITH_NICKEL = false.
Reintroduce global settings frontlight_intensity and
is_frontlight_on to keep level and state across koreader
sessions.
Fix a few of the remaining issues on kobo light.
Ensure settings are saved when rebooting/powering off from
File browser.
* Ensure untoggle works when starting with light off