Companion PR to https://github.com/koreader/koreader-base/pull/884
* Basically flags devices known to be stable when using PxP inversion.
* Plus, random fix for #4870 ;).
* A few FrontLight tweaks & cleanups on Kobo:
* Moved the Kobo-specific startup status insanity to Kobo-specific init
* Made turnOff/turnOn frontlight do a smooth ramp down/up
* On Kobo, use turnOff/turnOn for suspend/resume, to get that smooth toggle
* On Kobo, for NaturalLight w/ a mixer, only set warmth for setWarmth, and only set Brightness for setBrightness, otherwise, it tried to set both with not in-sync values, which made the FL widget jittery.
* Enforce a known rotation on startup, to make sure we handle touch input coordinates properly.
* Proper FrontLight warmth support (thanks to @cairnsh & @pazos in #4291)!
* Fix the PageTurn buttons mapping to match Nickel's defaults
* Properly remap PageTurn buttons depending on the current rotation.
* Actually enable the Mk.7 screen refresh codepath on *all* Mk.7 devices (I'd messed up the device check...).
* Full accelerometer handling (includes a touch of refactoring regarding orientation handling in general).
* Fix insidiously broken USBMS behavior in Nickel after we exit on FW >4.8.
Fix#4291Fix#3002