[#192] Fix variable redeclarations causing TypeError on 3.24

Created by: mwilck

GNOME Shell 3.24 now uses mozjs38 (SpiderMonkey 38) which has stricter syntax rules. https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey

In particular, it errors out on redeclarations of a variable inside of a code block.

Typical gnome-shell errors emitted by 3.24 look like this (unfortunately, no reference to the actually offending code in the extension except for the variable name):

JS ERROR: Exception in callback for signal: extension-found: TypeError: redeclaration of let fact initExtension@resource:///org/gnome/shell/ui/extensionSystem.js:221:5 loadExtension@resource:///org/gnome/shell/ui/extensionSystem.js:168:18 _loadExtensions/<@resource:///org/gnome/shell/ui/extensionSystem.js:304:9 _emit@resource:///org/gnome/gjs/modules/signals.js:126:27 ExtensionFinder<._loadExtension@resource:///org/gnome/shell/misc/extensionUtils.js:184:9 wrapper@resource:///org/gnome/gjs/modules/lang.js:178:22 bind/<@resource:///org/gnome/gjs/modules/lang.js:95:16 collectFromDatadirs@resource:///org/gnome/shell/misc/fileUtils.js:27:1 ExtensionFinder<.scanExtensions@resource:///org/gnome/shell/misc/extensionUtils.js:189:9 wrapper@resource:///org/gnome/gjs/modules/lang.js:178:22 _loadExtensions@resource:///org/gnome/shell/ui/extensionSystem.js:306:5 enableAllExtensions@resource:///org/gnome/shell/ui/extensionSystem.js:314:9 _sessionUpdated@resource:///org/gnome/shell/ui/extensionSystem.js:345:9 init@resource:///org/gnome/shell/ui/extensionSystem.js:353:5 _initializeUI@resource:///org/gnome/shell/ui/main.js:219:5 start@resource:///org/gnome/shell/ui/main.js:127:5 @:1:31

Fix this, and indicate gnome-shell 3.24 compatibility in metadata.json (not strictly needed any more, but may increase user confidence).