Initial Kindle PW5 support (#8856)

* Rejig frontlight warmth API to more closely match the existing API, and, hopefully, clarify some of its quirks, and reduce boilerplate and duplicate code in platform implementations.
* Tweak Kindle:setDateTime to prefer using the platform's custom script, as in interacts better with the stock UI. And make the fallbacks handle old busybox versions better.
* Add Kindle PW5 support ;).
* Add warmth support to the Kindle platform.
* Random TextBoxWidget cleanups: make sure we immediately free destroyed instances.
* FrontLightWidget: Refactor to make it slightly less obnoxious to grok and update; i.e., separate layout from update, and properly separate brightness from warmth handling. Move to simpler widgets instead of reinventing the wheel.
* TextBoxWidgets: Implement `setText` to match TextWidget's API, as some callers may be using the two interchangeably (i.e., Button).
* NaturalLightWidget: Make sure we pass a string to InputText
* InputText: Add debug guards to catch bad callers not passing strings ;).

frontend/device/sysfs_light.lua

@@ -53,6 +53,7 @@ dbg:guard(SysfsLight, 'setWarmth',
                      "Wrong warmth value given!")
           end)
 
+--- @note: warmth is already in the *native* scale!
 function SysfsLight:setNaturalBrightness(brightness, warmth)
     local set_brightness = true
     local set_warmth = true
@@ -67,8 +68,6 @@ function SysfsLight:setNaturalBrightness(brightness, warmth)
 
     -- Newer devices use a mixer instead of writting values per color.
     if self.frontlight_mixer then
-        -- Honor the device's scale, which may not be [0...100] (e.g., it's [0...10] on the Forma) ;).
-        warmth = math.floor(warmth / self.nl_max)
         if set_brightness then
             -- Prefer the ioctl, as it's much lower latency.
             if self.frontlight_ioctl then
@@ -77,7 +76,7 @@ function SysfsLight:setNaturalBrightness(brightness, warmth)
                 self:_write_value(self.frontlight_white, brightness)
             end
         end
-        -- And it may be inverted... (cold is nl_max, warm is nl_min)
+        -- The mixer might be using inverted values... (cold is nl_max, warm is nl_min)
         if set_warmth then
             if self.nl_inverted then
                 self:_write_value(self.frontlight_mixer, self.nl_max - warmth)