Please describe the problem:
Seahorse never shows the private key selection dialog when you are trying to
sign a key listed in seahorse main window.

Although in preferences, the default encryption key option is set to "None.
Prompt for a key.", it always uses the first added private key in your keyring,
and never shows the key selection dialog.

And even worse: if you choose any private key as a default one, it keeps using
the first added private key.

This is in contrast with the case when you are signing a document (e.g. in
nautilus): seahorse does - correctly - shows a key selection dialog in this case.

Steps to reproduce:
I have done the following test:

0. I backup my private keyring (secring.gpg in ~/.gnupg/)
1. Open seahorse from console (I have 2 personal keypairs created):
   Daniel 1 keypair (created first)
   Daniel 2 keypair (created after Daniel 1)
   Daniel 3 keypair (created after Daniel 2)
2. Import a (randomly choosed) public key from a keyserver --> correct
3. The imported public key shows in main window --> correct
4. Right click+sign option or Sign button on that key: the signing confirmation
   box for that key (signing all uids) appears --> correct
5. Accept all, leave default settings, confirm signing action --> correct

6. Seahorse shows the passphrase dialog for Daniel 1 ===> INCORRECT 
   I cancel signing the key

8. I delete Daniel 1, it gets deleted from main window --> correct
9. I close seahorse --> correct
10. I start seahorse from console --> correct
11. Repeat steps 4 and 5 for the same public key --> correct

12. Seahorse shows the passphrase dialog for Daniel 2 ===> INCORRECT 
   I cancel signing the key

13. I close seahorse --> correct 
14. I start seahorse from console --> correct
15. I import my saved private keyring into seahorse --> correct
    Now my Daniel 1 key appears again
16. I change the validity (and trust) for my imported key to ultimate --> correct
17. Repeat steps 4 and 5 for the same public key --> correct

18. Seahorse shows the passphrase dialog for Daniel 2 ===> INCORRECT 
   I cancel signing the key


Actual results:
Thus, seahorse always pick the first added private key when trying to sign
public keys in your keyring (seahorse's main window).


Expected results:
Seahorse should always show the key selection dialog / show passphrase dialog
for default key, in order to be consistent with preferences.


Does this happen every time?
This happens always.

Other information: