Highlight the hovered item in the task group tooltip

Summary:
Currently, the task group tooltip doesn't react to the user hovering over a thumbnail. This patch modifies the active window line to also act as a hover indicator, making the UI more responsive to the user's mouse movements.

Reviewers: Latte Dock, mvourlakos

Reviewed By: Latte Dock, mvourlakos

Subscribers: mvourlakos, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D19281