Simplify/change result type of KeyResolver and its internal helpers

KeyResolver:
* Use a single method for returning the result
* Remove the by-protocol mappings from the signing/encryption key result;
  the protocols of the keys are known by the keys

KeyResolverCore:
* Return a preferred solution and, if applicable, an alternative solution
  instead of hard to interpret by-protocol mappings

NewKeyApprovalDialog:
* Use preferred and alternative solutions of KeyResolverCore instead of
  merged key mappings
* In single-protocol mode use separate widgets for OpenPGP and S/MIME keys;
  this way the widgets keep the currently selected keys, when the user
  switches between OpenPGP and S/MIME
* In mixed-protocol mode use separate widgets for OpenPGP and S/MIME for
  the sender's signing and encryption keys and multi-protocols widgets
  for the recipients' encryption keys

GnuPG-bug-id: 5283