Add interfaces::ExternalSigner to let signer objects be passed between
processes and signer code to run in the original process, without
multiple processes linking and running signer code.
When no external signers are available, the option to enable external
signers should always be disabled. However the encrypt wallet checkbox
can erroneously re-enable the external signer checkbox. To avoid this,
CreateWalletDialog now stores whether signers were available during
setSigners so that future calls to external_signer_checkbox->setEnabled
can account for whether signers are available.