Misc: Get rid of the legacy defaults.lua globals (#9546)

* This removes support for the following deprecated constants: `DTAP_ZONE_FLIPPING`, `DTAP_ZONE_BOOKMARK`, `DCREREADER_CONFIG_DEFAULT_FONT_GAMMA`
* The "Advanced settings" panel now highlights modified values in bold (think about:config in Firefox ;)).
* LuaData: Isolate global table lookup shenanigans, and fix a few issues in unused-in-prod codepaths.
* CodeStyle: Require module locals for Lua/C modules, too.
* ScreenSaver: Actually garbage collect our widget on close (ScreenSaver itself is not an instantiated object).
* DateTimeWidget: Code cleanups to ensure child widgets can be GC'ed.

spec/unit/httpclient_spec.lua

@@ -1,11 +1,13 @@
--- set true to test httpclient
-DUSE_TURBO_LIB = false --luacheck: ignore
-
 describe("HTTP client module #notest #nocov", function()
     local UIManager
     setup(function()
         require("commonrequire")
         UIManager = require("ui/uimanager")
+        -- Set true to test httpclient
+        G_defaults:makeFalse("DUSE_TURBO_LIB")
+    end)
+    teardown(function()
+        G_defaults:delSetting("DUSE_TURBO_LIB")
     end)
 
     local requests = 0