Add support in nickel_conf to save/retrieve setting of
'autoColorEnabled', which is the automatic warmth feature in
Nickel. We do not support reading of 'BedTime', because it is encoded
as a QVariant. This setting is hence saved/loaded solely in/from
G_reader_settings.
* Use PluginShare to exchange data between plugins
* Remove legacy code in reader.lua and kobo/device.lua, and move KOBO_LIGHT_ON_START to kobo/powerd.lua
* A better sync config logic
* Consider kobo without hardware frontlight toggle
* update frontlight widget once toggle is tapped.
* Drops support for mocking the frontlight setting internally which may
cause incorrect in-memory values.
* Adds new supported value for `KOBO_LIGHT_ON_START` (-2), which sets
'Kobo eReader.conf' as the source to update `settings.reader.lua`'s
brightness setting on startup, thus using the value from it
indirectly.
* Adds the `KOBO_SYNC_BRIGHTNESS_WITH_NICKEL` configuration variable
which updates 'Kobo eReader.conf' every time the brightness setting is
changed within koreader.
* Fixes missing call to save brightness when modifying via two-finger
swipe.
Closes#1523.