Add a new design for notifications

[gcampax]

The current flow of notifications popping up more or less unannounced is very disruptive, and also not very useful if the agent speaks the notifications while nobody is listening. We might want to adopt a design similar to Alexa, in which all notifications are queued until the user asks to hear them. Of course, timer notifications (reminders, alarms, etc) would bypass this and hit the speech output directly.