yahoo protocol: Disconnect signals when login fails.

From the original ReviewBoard description:

Basically, sometimes, kopete is displaying in duplicate incoming
messages. Checking my log, I saw the following:

a) yahoo wasn't actually sending the message in duplicate.
b) but YahooACcount::slotGotIm() was called twice on that message
c) somewhere before the problematic thing - and I actually saw many
things being done in duplicate (not just messages), I saw this:

kopete(8352)/kopete (yahoo - raw protocol)
Client::slotLoginResponse: Emitting loggedIn

kopete(8352)/kopete (yahoo) YahooAccount::slotLoginResponse: -1 ,
"" )]

d) checking code in yahooaccount.cpp - it seems that for that code
(and others), it considers that account is offline, but forgets to
also disconnect all the signals. So, next time user logins (even by
an auto-reconnect), the signals are connected *again* to the same
sources. And that's why it gets to do things 2 times.

This commit contains the disconnection code without the message box
part, which includes new strings and will be committed only to trunk.

Original patch by Cristi Posoiu <cristi.posoiu AT gmail>, thanks!

REVIEW: 6421

FIXED-IN: 4.6.1