Highlight the icon in the system tray on mouse over

Summary:
Icon should be highlighted hover, as any other icons in the system tray.

CCBUG: 400295

Test Plan: Hover mouse over over the icon - it should highlight.

Reviewers: Plasma, davidedmundson, Plasma: Workspaces, broulik

Reviewed By: Plasma, Plasma: Workspaces, broulik

Subscribers: broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24738