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.

frontend/apps/reader/modules/readersearch.lua

@@ -12,6 +12,8 @@ local _ = require("gettext")
 local Screen = Device.screen
 local T = require("ffi/util").template
 
+local DGENERIC_ICON_SIZE = G_defaults:readSetting("DGENERIC_ICON_SIZE")
+
 local ReaderSearch = InputContainer:new{
     direction = 0, -- 0 for search forward, 1 for search backward
     case_insensitive = true, -- default to case insensitive