MainWindow: fix a bug where user/channel actions were enabled even though no user or channel was selected.
Fix incorrect handling of mumble:// links

If a user is connected and authenticated on a server,
and he/she clicks a mumble:// link that connects him/her
to the current server, he/she is moved to the channel
specified in the link. Previously, this would disconnect
and reconnect the user to the server.

If a user is connected to one server and follows a
mumble:// link to another server, he/she is now
placed in the channel specified in the link.
Previously, the channel they would be moved into
would be the channel they were in on the first
server.