(mime-raw-representation-type): New variable; abolish

(mime-raw-representation-type): New variable; abolish
`mime-text-decoder'.
(mime-text-insert-decoded-body): Change interface; refer
`mime-raw-representation-type' or `mime-raw-representation-type-alist'
instead of `mime-text-decoder' or `mime-text-decoder-alist'; abolish
function `mime-text-decode-buffer' and
`mime-text-decode-buffer-maybe'.
(mime-preview-text/plain): Modify for new interface of
`mime-text-insert-decoded-body'.
(mime-preview-text/richtext): Modify for new interface of
`mime-text-insert-decoded-body'.
(mime-preview-text/enriched): Modify for new interface of
`mime-text-insert-decoded-body'.