Better handling of resource collisions

[patrys]

Currently when two connections try to use the same resource name, both receive a notification from the server and get disconnected. The notification is not handled and the only way to detect such conflict is to use the XML console.

I propose that Gajim should detect such messages and display a notification popup with a message saying that there was a conflict detected. The notification window should have a 15 second timeout to reconnect with a button that aborts the process and opens the account settings allowing the user to change the resource name.

If you abort the process on your active session, the other session gets reconnected in 15 seconds without any interaction needed.

[asterix]

(In [8142]) when we're disconnected because of a resource conflict, ask the user another resource. Fixes #3044