Summary:
Window parts disappear very rapidly, it feels not really pleasant.
Animate also opacity so window parts disappear over time. This
makes fall apart animation more pleasant.

### Before

{F5912359}

### After

{F5912360}

Test Plan:
* Enabled fall apart effect
* Closed System Settings

Reviewers: #kwin, #plasma, #vdg, davidedmundson

Reviewed By: #kwin, #plasma, #vdg, davidedmundson

Subscribers: ngraham, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13528