This allows you to tell g_signal_connect what capability you care about when
connecting to the signals. This avoids your signal callback being called
multiple times and having to do a string comparison to check if it's the
capability you care about.
Added a g_warning to purple_ircv3_sasl_ack_cb and verified it only got called for sasl and not cap-notify.
Add detail to the ack and nak signals for PurpleIRCv3Capabilities