Implement "Unread on Top" in rooms list, with a toggle action.

Summary:
The only thing missing is saving/restoring this setting across ruqola
restarts.

Test Plan:
Checked the action, and now I see the following sections

Unread Rooms
Unread Private Messages
Favorites
Rooms
Private Messages

Pressing Esc in a room correctly moves it down to the bottom sections.

Reviewers: mlaurent

Reviewed By: mlaurent

Differential Revision: https://phabricator.kde.org/D21570