applets/notifications: re-implement critical notification line better

This commit re-implements the critical notification to be inside the
notification popup, not the notification item. This yields several
benefits:
- It's the semantically correct place to implement it since critical
  notifications don't appear in the history, so there's no reason to
  implement it in a component that's re-used for the history view
- The line extends down to the bottom for critical notifications that
  have  buttons, job progress, text input fields, files, or screenshots
- Simpler code overall