Messages notification try4

This PR consolidates previous attempts improvements inside the previous commits, so we don't spam git log with unnecessary intermediate commits.

Just to mention the improvements:
* extend notification body up-to 3 lines, as discussed in design meeting.
* simplified vertical space calculations, limiting it up-to 3 notifications, before the scrolled windowed activates.
* disconnect NotificationBox from notification model on destroy.
* use add_with_viewport instead of just add.
* display notification when clicking corner icon.