plugins: Add sliding notifications effect

Vlad Zahorodniirequested to merge
work/zzag/sliding-notifications into master

The new effect provides an eye candy animation when a notification appears or disappears.

In terms of UX, it's an improvement too because motion helps to capture user's attention.

Needs plasma-workspace!6526 (merged) so the animation looks good when one of several notifications is hidden.

Edited by Vlad Zahorodnii

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