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/readerui.lua

@@ -332,7 +332,7 @@ function ReaderUI:init()
             ui = self
         })
     end
-    self.disable_double_tap = G_reader_settings:readSetting("disable_double_tap") ~= false
+    self.disable_double_tap = G_reader_settings:nilOrTrue("disable_double_tap")
     -- back location stack
     self:registerModule("back", ReaderBack:new{
         ui = self,