canceling nickname change break various stuff

[misc]

Step to reproduce :

• enter a muc room, with more than one person
• try to change your nickname for one that is already taken
• when the dialog telling you to use a different nickname appear, just hit cancel
• try to change your presence

Expected result : no error message appear

Observed result : a message apear, and the xml sent by gajim is wrong

It seems gajim change the internal structure ( ie self.nick ) before the muc component send back the presence to ack the change, thuis leading to various problem. Here is a proposed patch, that place the new nick in a temporary member and that copy it once we have received a presence stanza.

[misc]

Proposed fix

[asterix]

(In [8904]) [misc] fix change nick behaviour. fixes #3495

[jim++]

(In [9089]) Readd part of [8904] removed in [8938]. Then mix them. So we fix #3495/#3506 and problem when connecting with an already used nick. Fixes problem when we change nick with dialog to an already used one and then cancel. Fixes #3506.

[asterix]

(In [9090]) [Jim] Readd part of [8904] removed in [8938]. Then mix them. So we fix #3495/#3506 and problem when connecting with an already used nick. Fixes problem when we change nick with dialog to an already used one and then cancel. Fixes #3506.