Instead of each effect, which needs to announce support, having custom
code to create a property and set it on the root window, there is now a
common API in EffectsHandler to take care of this.

The methods takes care of creating the atom if it has not already done
and set the property on the root window. Furthermore it allows multiple
effects to announce the same property without getting in conflict with
each other.

As a further convenience the property is automatically removed when the
effect is unloaded, so less things an effect author has to care about.

REVIEW: 107815