Add a "done" signal to PurpleIRCv3Capabilities so the connection knows when

capability negotiation has completed.
Add a "registration-complete" signal to PurpleIRCv3Connection. This can be used

by plugins to do additional startup commands before we tell libpurple that our

connection is connected. We'll need some more logic around emitting this signal

in the future, but this works for now.
Don't tell libpurple that our connection is connected until the class handler

for the new "registration-complete" signal is called so that libpurple doesn't

try to auto join any channels or anything similar before we're actually ready

for it to do so.

Connected and verified the changes with some g_messages that were removed.

Also used ngrep to verify that commands were still being sent correctly.