Summary: Let the ListView use its default highlighting (did I not notice before?) and keep the currently-selected chat highlighted at all times

Test Plan:
 - Launch app
 - Use mouse or arrow keys to select a conversation. The highlight should follow the selected conversation

Reviewers: #kde_connect, apol

Reviewed By: #kde_connect, apol

Subscribers: apol, kdeconnect

Tags: #kde_connect

Differential Revision: https://phabricator.kde.org/D17612