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 ;).

reader.lua

@@ -146,6 +146,7 @@ if dpi_override ~= nil then
     Device:setScreenDPI(dpi_override)
 end
 -- Night mode
+local hw_nightmode = Device.screen:getHWNightmode()
 if G_reader_settings:isTrue("night_mode") then
     Device.screen:toggleNightMode()
 end
@@ -347,6 +348,9 @@ local function exitReader()
     -- Close lipc handles
     ReaderActivityIndicator:coda()
 
+    -- Restore initial inversion state
+    Device.screen:setHWNightmode(hw_nightmode)
+
     -- shutdown hardware abstraction
     Device:exit()