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/ui/data/onetime_migration.lua

@@ -7,7 +7,7 @@ local lfs = require("libs/libkoreader-lfs")
 local logger = require("logger")
 
 -- Date at which the last migration snippet was added
-local CURRENT_MIGRATION_DATE = 20220914
+local CURRENT_MIGRATION_DATE = 20220922
 
 -- Retrieve the date of the previous migration, if any
 local last_migration_date = G_reader_settings:readSetting("last_migration_date", 0)
@@ -446,5 +446,34 @@ if last_migration_date < 20220914 then
     end
 end
 
+-- The great defaults.persistent.lua migration to LuaDefaults
+if last_migration_date < 20220922 then
+    logger.info("Performing one-time migration for 20220922")
+
+    local defaults_path = DataStorage:getDataDir() .. "/defaults.persistent.lua"
+    local defaults = {}
+    local load_defaults, err = loadfile(defaults_path, "t", defaults)
+    if not load_defaults then
+        logger.warn("loadfile:", err)
+    else
+        load_defaults()
+    end
+
+    for k, v in pairs(defaults) do
+        -- Don't migrate deprecated settings
+        if G_defaults:has(k) then
+            G_defaults:saveSetting(k, v)
+        end
+    end
+    G_defaults:flush()
+
+    local archived_path = DataStorage:getDataDir() .. "/defaults.legacy.lua"
+    local ok
+    ok, err = os.rename(defaults_path, archived_path)
+    if not ok then
+       logger.warn("os.rename:", err)
+    end
+end
+
 -- We're done, store the current migration date
 G_reader_settings:saveSetting("last_migration_date", CURRENT_MIGRATION_DATE)