Fade out OnScreenNotification when moving with the mouse over it

Summary:
This change triggers a fade out animation when the mouse is moved over
the OnScreenNotification, thus revealing the windows underneath. When
moving the mouse out of the OnScreenNotification it gets faded in again.

This is useful for various cases, e.g. when picking a color and the pixel
one wants to pick is underneath the notification, or when trying to kill
a window...

Test Plan: Manual testing in nested setup

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D3964