Bug 9580: (QA followup) add a Coce syspref and fix options for Providers

Bug 9580: (QA followup) add a Coce syspref and fix options for Providers

This proposed followup adds a 'Coce' syspref so it is consistent with
the rest of the enhanced content providers sysprefs (also, there's no
point making the user delete its configuration if disabling his Coce
integration for testing purposes, for example).

It also changes the CoceProviders syspref so it is a 'multiple' type
syspref. It puts some sensible descriptions to the options too.

Feel free to discuss this followup, it is just an idea!

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>
  - OK for the 'Coce' syspref: it's sensible to distinct configuration
    from enabling/disbaling the service.
  - OK also for the multichoices systempreference. If a new service
    popup, it won't be difficult to amend the .pref file.