Merge bitcoin-core/gui#555: Restore Send button when using external signer

2efdfb88aab6496dcf2b98e0de30635bc6bade85 gui: restore Send for external signer (Sjors Provoost)
4b5a6cd14967b8ec3cb525e4cb18628de6c15091 refactor: helper function signWithExternalSigner() (Sjors Provoost)
026b5b4523317fdefc69cf5cec55f76f18ad0c0a move-only: helper function to present PSBT (Sjors Provoost)

Pull request description:

  Fixes #551

  For the simplest use case of a wallet with one external signer and "PSBT Controls" disabled in settings (the default), the send dialog will behave the same as when using a wallet with private keys. I.e. there's no "Create Unsigned" button.

  When PSBT controls are turned on, you can now actually make a PSBT with signing it; before this PR that button would trigger a sign event and would broadcast the transaction.

  In case a multisig, the Send button will sign on the device, and then fall back to presenting a PSBT (same behavior as before #441).

  This PR starts with two refactoring commits to move some stuff into a helper function for improved readability in general, and to make the main commit easier to review.

ACKs for top commit:
  jonatack:
    utACK 2efdfb88aab6496dcf2b98e0de30635bc6bade85 diff review since my last review, code re-review, rebased to current master, verified clean debug build of each commit
  luke-jr:
    utACK 2efdfb88aab6496dcf2b98e0de30635bc6bade85

Tree-SHA512: e8731a0ef9e87564b2676c7b022b742d9621bba964c19dba9fd9f6961eb608737a9e1a22c0a3c8b2f2f6d583bba067606ee8392422e82082deefb20ea7b88c7c

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