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/reader/modules/readerstyletweak.lua

@@ -327,7 +327,7 @@ function ReaderStyleTweak:updateCssText(apply)
 end
 
 function ReaderStyleTweak:onReadSettings(config)
-    self.enabled = not (config:readSetting("style_tweaks_enabled") == false)
+    self.enabled = config:nilOrTrue("style_tweaks_enabled")
     self.doc_tweaks = config:readSetting("style_tweaks") or {}
     -- Default globally enabled style tweaks (for new installations)
     -- are defined in css_tweaks.lua
@@ -342,7 +342,7 @@ function ReaderStyleTweak:onSaveSettings()
     if self.enabled then
         self.ui.doc_settings:delSetting("style_tweaks_enabled")
     else
-        self.ui.doc_settings:saveSetting("style_tweaks_enabled", false)
+        self.ui.doc_settings:makeFalse("style_tweaks_enabled")
     end
     self.ui.doc_settings:saveSetting("style_tweaks", util.tableSize(self.doc_tweaks) > 0 and self.doc_tweaks or nil)
     G_reader_settings:saveSetting("style_tweaks", self.global_tweaks)