applets/notifications: show critical notification line horizontally
The line recently added to critical notifications suffered from a few issues:
- The line height depended on the content height, so it would be quite small for small notifications, impairing the primary goal of being highly visible
- By going into the bottom-left corner, it worsened the pre-existing bug of plasma dialog corners not being appropriately rounded
- Due to an oversight in the prior work, it didn't extend all the way down to the bottom when the notification had buttons, job progress, or thumbnails
Attempts to fix the last issue through code proved too challenging, and would not have fixed the others anyway. So this commit changes around the UI a bit to show the line directly under the header as a horizontal strip, not a vertical one.
