Use :transient desktop notifications

I'm using the following to enforce transience in all cases, but I think `ement-notify--notifications-notify` should either be doing this by default, or else that it should be a user option.

In my case it's as much a Gnome problem as anything else (it has a hard-coded limit for the number of persistent notifications after which subsequent notifications are rejected with an error), but my feeling is that transient would be a sane default for ement because it provides its own log buffer for persistent notifications.

```el
;; Notifications -- force transience.
;; Otherwise I need to regularly run 'gnome-panel', purge its
;; notifications, and then kill 'gnome-panel' again.
(define-advice notifications-notify (:filter-args (params) transient)
"Force freedesktop notifications to be transient.

Advice for `notifications-notify'. To remove:
\(advice-remove \\='notifications-notify \\='notifications-notify@transient)"
(plist-put params :transient t))
```