Make warning message warnier
A collection of small changes to increase the effectiveness of this message:
- Since it's a warning, use the semantically correct warning message type
- Front-load the important part of the text
- Specifically mention that stuff here hasn't been reviewed for safety
- Specifically mention that stuff kete hasn't been reviewed by KDE
In the future I'd also like to make this message even more warnier for content that we know can ship executable code, and less so for content that's just asets and metadata. But the plumbing for that isn't in place yet.