individual-menu: use the proper persona in submenus

'single_individual' uses to be a FolksIndivudal containing a single persona
(the one from the submenu) ensuring that operations (chat, call, etc) were
performed on this very persona.

But we broke this in 0909d1f8 by replacing the
empathy_create_individual_from_tp_contact() call by
empathy_ensure_individual_from_tp_contact() as the same individuals (the one
will all the personas) was now used in all the sub menus.

Fixing this by introducing 'contact' variant of the menu_item_new methods
which is always cleaner.

https://bugzilla.gnome.org/show_bug.cgi?id=732100