Session of numeric events passed to text_emit() for nicks with open query dialogs

Now process_numeric() passes the right session context to text_emit() when an
numeric event occurs regarding a nick with a open dialog.

Since this session context is potentially passed to plugins this can change the
behavior of plugins, but I think this is the intended semantics
(http://docs.hexchat.org/en/latest/script_python.html#context-theory).

Another consequence is that event messages will be displayed in the query dialog
of the nick in question (if a query dialog is open, of course), which I think is
sensible. (The initial intention of this patch was actually to report the
"No such nick" message in the query dialog when sending a message to a user that
has quit.)