systray: show a small animation on the icons when activated

Summary: Both for plasmoids and SNI

Test Plan: Now there's some kind of feedback when pressing the icons

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: filipf, broulik, anthonyfieroni, davidedmundson, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19947