#123 Visually distinguish notifications for private messages more easily

Currently in the notifications area, Halcyon distinguishes direct messages through a different set of icons for the individual toots. Coming from Mastodon, and considering they have established a visual language that makes it very easy to distinguish individual direct toots, it could be possible to consider adding this little tweak here, too. As such it would create less cognitive overhead to distinguish the notifications by kind and thus enable us to react to them more easily. Considering a filtered view of the notifications, it could also make sense to think about breaking out direct messages into a separate direct messages pane, similar to what Twitter do.