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/ui/widget/container/widgetcontainer.lua

@@ -118,11 +118,12 @@ function WidgetContainer:handleEvent(event)
     end
 end
 
-function WidgetContainer:free()
+-- Honor full for TextBoxWidget's benefit...
+function WidgetContainer:free(full)
     for _, widget in ipairs(self) do
         if widget.free then
             --print("WidgetContainer: Calling free for widget", debug.getinfo(widget.free, "S").short_src, widget, "from", debug.getinfo(self.free, "S").short_src, self)
-            widget:free()
+            widget:free(full)
         end
     end
 end