Fix / Improve handling of overrides in Keyapproval

Now the keyresolver resolves overrides for both protocols
and passes both the key approval dialog. The approval
dialog then splits them up and hides the combos that
were created by overrides.

This fixes switching the dialog between OpenPGP and
S/MIME in exclusive mode when mutliple overrides
are selected.

GnuPG-Bug-Id: T4479