QueryBufferItem disconnect IrcUser when removing

QueryBufferItem disconnect IrcUser when removing

Add call to disconnect() when removing IrcUser from QueryBufferItem,
preventing _ircUser from triggering removeIrcUser() again when it's
destroyed by going out of scope.  Additionally, limit dataChanged()
to when IrcUser is removed.  May offer slight speed benefits.

Fixes at least one case of the client showing some nicknames as
offline when they're actually available (e.g. restarting the client
without restarting the core reveals them as online).

Test case from Bitlbee - receive following (no delay between AWAYs):
:nick!name@gmail.com JOIN :&bitlbee
:nick!name@gmail.com AWAY :Away (Testing)
:nick!name@gmail.com QUIT :Leaving...
:nick!name@gmail.com AWAY :User is offline

Before, 'nick' was wrongly shown as not available.  After, 'nick' is
shown as away with the message "User is offline".

(Bitlbee does this as GTalk/XMPP allow for messaging offline users)

Resolves GH-239.

(cherry picked from commit 5f81caea6345ed85a91e2d03d5a626081392d12c)