Implement adding subkeys to an existing key

GnuPG-bug-id: T6877

The new dialog for choosing algorithm, expiry, etc. is largely based on the existing "new certificate" / "advanced settings" dialog. Unfortunately, the behavior needs to be somewhat different, so I don't think it's feasible to adapt the existing dialog to support the new use-case in the same codebase. This means quite a bit of code duplication for now.

I'll try to reduce this later, so don't bother reviewing the code yet.

@aheinecke I'd appreciate if you could try running it and tell me whether it's generally how you want it to work, before I spend more time improving the code.