[SystemTray] Support for AttentionIcon

Summary:
Adding support for Attention Icon to StatusNotifier tray icons

BUG: 341255

Test Plan: As described in bug report, Konversation can be used for tests.

Reviewers: Plasma, davidedmundson, apol, mart, ngraham

Reviewed By: Plasma, davidedmundson, ngraham

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24767