[scripting] Allow effects to grab windows

Summary:
Some JavaScript based effects need to grab particular windows in order
to avoid conflicts with other effects.

Example usage:

```lang=js
effects.windowAdded.connect(function (window) {
    if (effect.grab(window, Effect.WindowAddedGrabRole)) {
        window.coolWindowTypeAnimation = animate({
            ...
        });
    }
});
```

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: romangg, graesslin, davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13153