Disable all extensions in reverse order

So we avoid having to disable and enable an extension that is to
be disabled later anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=700784

js/ui/extensionSystem.js

@@ -292,7 +292,7 @@ function disableAllExtensions() {
         return;
 
     if (initted) {
-        enabledExtensions.forEach(function(uuid) {
+        extensionOrder.slice().reverse().forEach(function(uuid) {
             disableExtension(uuid);
         });
     }