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/apps/reader/modules/readerfooter.lua

@@ -160,7 +160,7 @@ local footerTextGeneratorMap = {
         local prefix = symbol_prefix[symbol_type].frontlight_warmth
         local powerd = Device:getPowerDevice()
         if powerd:isFrontlightOn() then
-            local warmth = powerd:getWarmth()
+            local warmth = powerd:frontlightWarmth()
             if warmth then
                 return (prefix .. " %d%%"):format(warmth)
             end