Refactoring: Create NotificationUtil class
Use notification instead of toast on module updates

Also give Android 4.1+ users the possibility to reboot right from the
notification.

The toast message is still shown when the user enables/disables a module
because it happens when the user is already in the app. I think posting a
notification would rather confuse than help people.
Cancel notifications in a more targeted way

Notifications stay now until the user entered the correct screen (or
swiped them away). Well, at least that was the goal. Going to the modules
section still cancels all notifications, as it's not possible to cancel
all notifications with a specific ID and any tags. If really required, we
would need to keep a list of all "module not enabled" notifications, but
it doesn't seem worth the effort and complexity at this point.