Set transient hint in notifications

On [Debian bug #738887](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=738887), Hank Donnay writes:

> If mpDris2 is run in GNOME3 with notifications enabled, the
> notifications will stay in the Message Bar until dismissed.
>
> The included patch should add the 'transient' hint to notifications, in
> line with the [GNOME Desktop Notifications Spec](https://developer.gnome.org/notification-spec/)

Here's his patch as a git commit.