individual-menu: rename menu_item_new functions

- Remove the namespace prefix for static functions
- Add 'individual' suffix

https://bugzilla.gnome.org/show_bug.cgi?id=732100
individual-menu: factor out some item_new() code

https://bugzilla.gnome.org/show_bug.cgi?id=732100
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 0909d1f884c4542553d98b18e7622664d2a02169 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
individual-menu: remove 'block' item from personas sub menus

Blocking a specific persona from an individual is a very uncommon operation.
Either you want to block the full individual or the personas shouldn't be
linked together.
There is not much point cluttering the menu with it.

https://bugzilla.gnome.org/show_bug.cgi?id=732100
individual-menu: stop creating single_individual

This code was always pretty hacky and created loads of nasty bugs so I'm glad
to get rid of it.

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