Implement notification for missed call, fixes #52

At the end, it was easier than it seemed initially...

As usually, feel free to re-implement this in a better way or take over if you see a quicker way to implement this feature!

Todo:

• have the notification triggered only for missed calls (incoming, not rejected and duration=0)
• show CallerId
• add "Call back" button in the notification
• when there are more than one missed calls, substitute the existing notification for the 1st missed call by just showing the number of missed calls without CallerId
• un-comment relevant lines in translation files (at least in the Italian translation, those are translated but commented)