Do not allow overwriting bare jid of myself account in jabber client

Kopete account id and myself contact id cannot be changed. This means that all
jabber code must use bare jid for myself contact specified in account dialog.
Jabber server can change user jid after successfull login, but there is no
way in Kopete to propage it. So allow changing only of resource name and do not
tell other Kopete code about bare jid changes (which is account id and myself
contact id). Chaning resource name is OK. Without this patch it is not possible
to login to jabber servers which chaning bare jid (e.g facebook).

BUG: 324937
FIXED-IN: 4.11.3