Make use of AppImage signatures

[TheAssassin]

Some AppImages are signed by their authors. AppImageLauncher should make use of those data, e.g., by displaying the author of an AppImage, and whether the signature applies (or alternatively refuse integration if the signature is invalid, similar to how AppImageUpdate works in these cases).

[azubieta]

I guess that signature management code should go into libappimage so it will become available to the rest of the tools. I guess that using a separated header for this will be a good idea.