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/textwidget.lua

@@ -312,10 +312,12 @@ function TextWidget:getBaseline()
 end
 
 function TextWidget:setText(text)
-    if text ~= self.text then
-        self.text = text
-        self:free()
+    if text == self.text then
+        return
     end
+
+    self.text = text
+    self:free()
 end
 dbg:guard(TextWidget, "setText",
     function(self, text)