Provide a warning if user tries to open Zimit-based archive

[Jaifroid]

Kiwix Desktop currently provides a warning if a user tries to open a Zimit-based archive, It occurred to me we should do something similar in this app (and also ensure the app doesn't just hang when trying to open such a file). I have experience in detecting a Zimit archive reliably.

[mossroy]

If you can detect that reliably, that's a good idea