keyboard: Update OSK key model from JSON to new format

Every key is now a map able to handle more properties, so its behavior
can be extended. For now, minimally adapt to the new format so the OSK
does not break along the update.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2278>

js/ui/keyboard.js

@@ -1511,7 +1511,8 @@ var Keyboard = GObject.registerClass({
     _addRowKeys(keys, layout) {
         for (let i = 0; i < keys.length; ++i) {
             let key = keys[i];
-            let button = new Key(key.shift(), key);
+            const {strings} = key;
+            let button = new Key(strings.shift(), strings);
 
             /* Space key gets special width, dependent on the number of surrounding keys */
             if (button.key == ' ')