Various improvements to the Jabber "join chatroom" dialog:
- place the "nick" line edit on the top of the dialog
- add a "query" button next to the server line edit, and a list with them below it; this way, the separate dialog for browsing them is no more needed
- add a "cancel" button for closing the dialog
- make the service scanning working, by switching from a JT_GetServices task to a JT_DiscoItems one (like in the "service" browser dialog)
- disable the "join" button if at least one of nick, server and chatroom line edits is empty
- double click on a chatroom writes it in the chatroom line edit, and if nick and server ones are not empty, the "join" action is started