This makes the api of EffectWindow more similar to the api of Window. It
also makes more sense to keep such signals in EffectWindow. In the future,
the effect window can be dropped in favor of the window.
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
