Activity reminder notification should use transient hint

Created by: dylanmccall

Hamster occasionally reminds me about my current activity. Using GNOME Shell, this notification stays in the message tray until I close it, which is kind of annoying and distracting — especially when I have since acted on the notification. Hamster should add the "transient" hint to its notifications. This way, the shell will show a notification banner for a few seconds, but the notification will not persist after the banner disappears.