(TopLevel): Require `mime' and `eword-deocode' instead of `mmbuffer'.

(TopLevel): Require `mime' and `eword-deocode' instead of `mmbuffer'.
(mmgnus-entity): Rename from `mime-gnus-handle-entity' and use `mime-entity'
as parent class.  Add slot `body' and `header'.
(mmgnus-visible-field-p): New function. (copy from `mime-visible-field-p' in
FLIM's mmbuffer.el)
(mmgnus-insert-header-from-string): New function.
(mime-insert-header): Define new method.
(mime-entity-content): Likewise.
(mime-gnus-entity): Use `mmgnus-entity' as parent class.