Bug 864824 - Context should be able to switch to a new set of locales. r=gandalf

For the use case of the user switching the language, there should be a way to
change the locales available in the context.

 - Rename GlobalsMnager to RetranslationManager
 - Relax the requirements for calling addResource & linkResource

   The summary of the (add|link)Resource logic is as follows (`y` means "the
   method adds resources to this locale or these locales"):

                                 _none   _locales
        addResource                y
        linkResource(string)       y         y
        linkResource(function)               y

 - Make get and getEntity synchronous only
 - Context.localize() should return an object, for future-compatibility
 - Context.registerLocales() may call freeze() if the context is frozen and
   ready;  you don't need to call freeze yourself.