Now we check that we can actually make an ssh_key out of it, and
moreover, that the key is of a sensible kind (i.e. not a certificate
in turn). If that's not true, we report something about the problem in
a new CTRL_TEXT below the public key input box. If the key _is_ valid,
that same text control is used to show its type, length and
fingerprint.
On Windows, I've widened the dialog box a little to make fingerprints
fit sensibly in it.
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