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).

spec/unit/luadata_spec.lua

@@ -45,7 +45,7 @@ describe("luadata module", function()
 
         Settings:saveSetting("key", {
             a = "b",
-            c = "true",
+            c = "True",
             d = false,
         })
 
@@ -55,7 +55,7 @@ describe("luadata module", function()
         assert.True(child:has("a"))
         assert.are.equal(child:readSetting("a"), "b")
         assert.True(child:has("c"))
-        assert.True(child:isTrue("c"))
+        assert.False(child:isTrue("c")) -- It's a string, not a bool!
         assert.True(child:has("d"))
         assert.True(child:isFalse("d"))
         assert.False(child:isTrue("e"))