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.
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