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/button.lua

@@ -96,7 +96,7 @@ function Button:init()
                 local new_size = self.label_widget.face.orig_size - 1
                 if new_size < font_size_2_lines then
                     -- Switch to a 2-lines TextBoxWidget
-                    self.label_widget:free()
+                    self.label_widget:free(true)
                     self.label_widget = TextBoxWidget:new{
                         text = self.text,
                         line_height = 0,
@@ -118,7 +118,7 @@ function Button:init()
                 if new_size < 8 then -- don't go too small
                     break
                 end
-                self.label_widget:free()
+                self.label_widget:free(true)
                 self.label_widget = TextWidget:new{
                     text = self.text,
                     max_width = max_width,