Port the KWallet Keyring to the new CredentialProvider API.

Testing Done:
* connected account with wallet locked, verified that we weren't prompted for a password until the wallet was unlocked.
* connected account, didn't save password, made sure it connected and wasn't stored in kwalletmanager5
* connected account, saved password, made sure it connected and verified the password was stored in kwalletmanager5
* reconnected account, made sure the account connected without prompting
* reopened pidgin, made sure the account connected without prompting.
* disconnected pidgin from kwalletmanager5, re-connected account, verified it reconnected via debug and kwalletmanager5
* force closed the wallet in kwalletmanager5, re-connected account, unlocked wallet, verified no password prompts and that the account connected.
* removed the account and verified the password was removed from kwalletmanager5

Bugs closed: PIDGIN-17488

Reviewed at https://reviews.imfreedom.org/r/575/