wallet: sign_message: strip whitespaces in GUis, do not strip in CLI

[SomberNight]

(stacked on top of #10790, which is mostly clean-up, merge that first)

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The design decision to make is if we stand-by the stance from #4327:

In the command line, having whitespaces is very explicit, the user is fully aware;
unlike in Qt, where it's not so clear, so it's better to strip there.

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• stripping whitespaces leads to better UX for GUIs IMO, but it's counter-productive for CLI
• our previous behaviour was unintentionally inconsistent:
  • qt stripped the message in both sign and verify
  • qml stripped the message in verify but not in sign (until qml: strip whitespace from message before signing, as in qt gui #10787)
  • cli stripped the message in both sign and verify
  • qml also stripped the signature in verify, which was not done anywhere else
• now this code is de-duped, and the default becomes stripping both message and signature; with the CLI explicitly opting out of that

closes #10787
closes #10788

[SomberNight]

The design decision to make is if we stand-by the stance from #4327

I think we stand-by that.
Hence this PR does not change the intent of the code and is just a clean-up.