Show notification and play sound for new messages on Linux

Still WIP since missing libnotify or even notification server is not tested nor handled appropriately yet: As per #156 (comment 67055) we want to inform the user that they should install libnotify for notifications to work.

Also:

• current "new" message count does not include unread messages at application start, but still decrements if those are read. We could discuss if we wanted to change wording from "new" to "unread" and include those as well (related to #133 (closed) to get a sum for all private messages) > I didn't change wording (yet), but we do include unread messages at application start now as well
• on Android, despite saying "x new private messages", the implementation is actually counting the number of contacts with unread messages. We should decide which way makes more sense. We could also add more information and say "x new messages in y private chats"? actually everything works as expected on Android
• needs to be tested with different notification servers (see the list here) > has been manually tested on a KDE Plasma and Gnome desktop and confirmed to work fine

Successfully tested on Debian/Xfce with xfce4-notifyd and libnotify installed:

Briar-notification

Closes #156 (closed), #310 (closed)