Returns a GtkTreeModel of header names and values and visibility flags,
built from the CamelMimeMessage. The tree model rows can be reordered
and toggled prior to printing.
Also add e_mail_part_headers_is_default() as a handy helper.
This will replace the headers API in EMailFormatter. Need a more
permanent place for headers since EMailFormatter is too disposable.
Also add an ESettingsMailPartHeaders class, which binds the new property
to the "show-headers" setting with a suitable mapping function to filter
out disabled header names.
This is a weak reference to the EMailPartList to which the EMailPart
has been added. The property is set by e_mail_part_list_add_part().
New functions:
e_mail_part_ref_part_list()
e_mail_part_set_part_list()
