EMFormatHTML now holds a sealed EWebView instead of a public GtkHTML,
accessible through em_format_html_get_web_view().

Rename e_mail_reader_get_html_display() to e_mail_reader_get_formatter()
and have it return an EMFormatHTML instead of an EMFormatHTMLDisplay,
since that's usually the type you want (or else an EMFormat, but never
an EMFormatHTMLDisplay).