Improve the main notification texts

With the new notification design we should also check all the texts and make them easier to understand and as compressed as we can.

The new main notifications have also titles now. They should already be meaningful enough for user to understand what is the notification about. 

Tasks:

• Create titles for all the main notifications
• Make body texts as clear and compressed as possible.
• Check that button texts are unified and as compressed as they can.

List of notifications:https://intranet.qt.io/display/QTPO/Notifications