[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:

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