closeDialog: Actually set key focus to button when grabbing key focus

When setting the key focus to `this._dialog`, the default button is not
automatically focused and no button has key focus.

Use the `initialKeyFocus` property of the dialog instead, and set focus
to the default button if the dialog is not already focused.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/786>
(cherry picked from commit de834fe307)

js/ui/closeDialog.js

@@ -208,7 +208,11 @@ export const CloseDialog = GObject.registerClass({
     }
 
     vfunc_focus() {
-        if (this._dialog)
-            this._dialog.grab_key_focus();
+        if (!this._dialog)
+            return;
+
+        const keyFocus = global.stage.key_focus;
+        if (!keyFocus || !this._dialog.contains(keyFocus))
+            this._dialog.initialKeyFocus.grab_key_focus();
     }
 });