Invitations should disappear once a MUC is joined

A common situation:

• I receive a group chat invitation
• Conversations automatically joins the group
• Via bookmarks, this triggers gajim to join the group too
• I still have the notification for the groupchat invitation in gajim

I think gajim should just dismiss invitations notifications for a MUC as soon as joins it. The only drawback I see to this is that you might lost a message (the "reason") associated with the invitation. If this is a major issue, maybe we should have a way to view history of such events once their notification is dismissed. An option would be to display the invitation in the MUC itself, eg:

18:14 - You have been invited to this group by XXX. They gave this reason: "You should join this group!".