Implement per-contact notifications

[valerio]

KDE's KNotification, the KDE class to show passive notifications, allows us (using so-called Contexts) to set per-contact or per-group notifications.

We should definitely take the opportunity ;)  Here's the API reference.

 Topic reference

[valerio]

To clarify add up on that, the best idea imho is to allow setting up global notifications, then offer for each contact an overriding options, with a fully customized event editor, to change the events just for that contact.