[effects/dialogparent] Fix strict mode issues

Summary:
Modification of undeclared variables is illegal in the strict mode, e.g.

```lang=js
mainWindows = window.mainWindows();
```

is illegal.

Test Plan: The Dialog Parent effect works with both QScriptEngine and QJSEngine.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15413