Room list: use gray text color for rooms with no unread messages

Summary:
This way, the rooms with unread messages stand out in a different
color, just like in RC+. This helps noticing them better on systems
(like my hi-dpi linux) where a bold font is barely noticeable.

Test Plan: tested ruqola with both default and dark color schemes.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: Ruqola

Differential Revision: https://phabricator.kde.org/D21487