AppImages can now be signed with a GPG signature key

[Claus Dietrich <claus.dietrich@xxxxxxxxxx>]

Hi Benoit

I found an exciting master commit from Tuesday this week which now 
allows Gambas AppImages to be signed with a GPG signature key. So I 
tried it and it works great. This new feature will help to distribute 
Gambas applications.

Thanks a lot!

Best regards

Claus