Partial fix of issue #1190.  If a user follows a mumble:// link to a server
he/she is already connected to, we do not want to reconnect if:
- the link had a different username than what he/she is connected as, and
- the user is registered on the server
Partial fix of issue #1190.  If a user follows a mumble:// link to a server
that is not the server that he/she is currently connected to, he/she is now
moved into the channel that was specified in the link.

The root cause was that qsDesiredChannel was being overwritten in
MainWindow::serverDisconnected.