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
8593823f6c