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/readerlink.lua

@@ -14,6 +14,7 @@ local Notification = require("ui/widget/notification")
 local QRMessage = require("ui/widget/qrmessage")
 local UIManager = require("ui/uimanager")
 local ffiutil = require("ffi/util")
+local lfs = require("libs/libkoreader-lfs")
 local logger = require("logger")
 local util = require("util")
 local _ = require("gettext")
@@ -465,7 +466,7 @@ function ReaderLink:showLinkBox(link, allow_footnote_popup)
         if sbox then
             UIManager:show(LinkBox:new{
                 box = sbox,
-                timeout = FOLLOW_LINK_TIMEOUT,
+                timeout = G_defaults:readSetting("FOLLOW_LINK_TIMEOUT"),
                 callback = function()
                     self:onGotoLink(link.link, false, allow_footnote_popup)
                 end