[KScreen Effect] Fix fade to black

Summary:
When you have two windows ontop of each other and you turn them
transparent, evidently you'll be able to see through them.
Not only does it look unpolished if the desktop window flashes through
your windows, it can pose a privacy risk as you'd be able to briefly
look through the lock screen.

This patch fades apps to black intead of to transparent. As this looks
weird for panels, so windows are also faded to opaque (if relevant) at
the same rate.

BUG: 388384

Reviewers: #plasma, graesslin

Subscribers: luebking, plasma-devel, kwin, #kwin

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9608