EMailExtension is now too trivial to keep as a standalone interface.
Add a 'mime_types' string array to the EMailFormatterExtension and
EMailFormatterParser interface structs.

Alter e_mail_extension_registry_add_extension() to take a 'mime_types'
string array and the GType of an extension to instantiate, rather than
the extension instance directly.

e_mail_extension_registry_remove_extension() is no longer needed.