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/pluginloader.lua

@@ -124,7 +124,7 @@ function PluginLoader:loadPlugins()
                         table.insert(self.enabled_plugins, plugin_module)
                     end
                 else
-                    logger.dbg("Plugin ", mainfile, " has been disabled.")
+                    logger.dbg("Plugin", mainfile, "has been disabled.")
                 end
                 package.path = package_path
                 package.cpath = package_cpath
@@ -203,7 +203,7 @@ function PluginLoader:createPluginInstance(plugin, attr)
         self.loaded_plugins[plugin.name] = re
         return ok, re
     else  -- re is the error message
-        logger.err("Failed to initialize", plugin.name, "plugin: ", re)
+        logger.err("Failed to initialize", plugin.name, "plugin:", re)
         return nil, re
     end
 end