GPGME can be used to get secret key IDs from the keyring, and a GtkComboBoxEntry could be used to make them available for selection. With GtkComboBoxEntry, user can still type the key ID directly.
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Created attachment 1892 [details] In account prefs, show available secret keys in a combo box. v1 Ready for testing and suggestions - primarily about what else to display for each key in combobox selection. I'm thinking at least key expiry date.
Created attachment 1898 [details] In account prefs, show available secret keys in a combo box. v2 Changes from v1: - removed extraneous newlines in combobox options - do not include revoked, disabled or invalid keys in combobox options - use long (64-bit) key ID for OpenPGP keys in combobox options's user visible string - include expiration date in the user visible string - if user enters a custom long or short key ID, select matching key (if any) from among combobox options when focus leaves the entry box