LuaSettings: Add a method to initialize a setting properly (#7371)

* LuaSettings/DocSettings: Updated readSetting API to allow proper initialization to default.
Use it to initialize tables, e.g., fixing corner-cases in readerFooter that could prevent settings from being saved.
(Fixes an issue reported on Gitter).
* LuaSettings/DocSettings: Add simpler API than the the flip* ones to toggle boolean settings.
* Update LuaSettings/DocSettigns usage throughout the codebase to use the dedicated boolean methods wher appropriate, and clean up some of the more mind-bending uses.
* FileChooser: Implement an extended default exclusion list (fix #2360)
* ScreenSaver: Refactor to avoid the pile of kludges this was threatening to become. Code should be easier to follow and use, and fallbacks now behave as expected (fix #4418).

frontend/apps/cloudstorage/cloudstorage.lua

@@ -32,6 +32,7 @@ local CloudStorage = Menu:extend{
 }
 
 function CloudStorage:init()
+    --- @todo: Probably a good candidate for the new readSetting API
     self.cs_settings = self:readSettings()
     self.show_parent = self
     if self.item then