Explicitly convert raw data to strings

As strings are guaranteed to use UTF-8 in the GNOME platform, generic
file APIs like g_file_load_contents() return raw data instead. Since
gjs' recent update to mozjs60, this data is now returns as Uint8Array
which cannot simply be treated as string - its toString() method boils
down to arr.join(',') - so use gjs' new ByteArray module to explicitly
convert the data.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/179

js/ui/keyboard.js

@@ -472,6 +472,8 @@ var KeyboardModel = new Lang.Class({
     _loadModel(groupName) {
         let file = Gio.File.new_for_uri('resource:///org/gnome/shell/osk-layouts/%s.json'.format(groupName));
         let [success, contents] = file.load_contents(null);
+        if (contents instanceof Uint8Array)
+            contents = imports.byteArray.toString(contents);
 
         return JSON.parse(contents);
     },