Discard Deleted before Scene is destroyed

Summary:
Deleted has lifetime bounded to effects and scene.

We can't discard Deleted before EffectsHandler is destroyed because
effects that referenced it may call unrefWindow.

On the other hand, the fact that Deleted may outlive scene doesn't
make sense because Deleted exist purely for animation purposes and
nothing more.

This change arranges lifetime of Deleted so it's more reasonable.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: graesslin, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18914