Now we have the following extension points in the GType hierarchy:

   E_TYPE_MAIL_FORMATTER_EXTENSION
   E_TYPE_MAIL_FORMATTER_PRINT_EXTENSION
   E_TYPE_MAIL_FORMATTER_QUOTE_EXTENSION
   E_TYPE_MAIL_PARSER_EXTENSION

A registry just needs to be given one of these extension points, and it
can use g_type_children() to find and load all registered extensions.

This eliminates e-mail-format-extensions.[ch] as well as the dynamic
loaders I added a few commits back.  Dynamically loaded extensions are
now easier to register, at the cost of internal extensions being a tad
more cumbersome to register.  Fair tradeoff, imo.

This also makes e_mail_extension_registry_add_extension() a private
function used only by e_mail_formatter_extension_registry_load() and
e_mail_parser_extension_registry_load().