[SystemTray] Support for AttentionIcon

Summary:
Adding support for Attention Icon to StatusNotifier tray icons.
Binding is working correctly only when "Status" is used, not derived "status".

BUG: 341255

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

Reviewers: #plasma, broulik, #plasma_workspaces

Reviewed By: #plasma, broulik, #plasma_workspaces

Subscribers: lbeltrame, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24865