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
