[applets/taskmanager] Add highlight effect for hovered tooltip

Summary: Looks better now, and matches the hover effect for the close button.

Test Plan: {F7816666}

Reviewers: #plasma, #vdg, mart

Reviewed By: #plasma, #vdg, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25959