Delete RCS cookie.

Delete RCS cookie.
(mime-editor/RCS-ID): Deleted.
(mime-editor/version): Don't use `get-version-string' and
`mime-editor/RCS-ID'.
(mime-content-types): Add `png'.
(mime-file-types): Add setting for *.htm, *.jpeg, *.png, *.tif, *.txt
and *.diffs; don't specify "7bit" for *.el.
(mime-editor/menu-list): Add "Insert File (verbose)".
(mime-editor/define-menu-for-xemacs): Use `easy-menu-add'.
(mime/editor-mode): Add description for
`mime-editor/insert-file-verbose'.
(mime-editor/insert-file): Call `mime-prompt-for-encoding' if encoding
is nil.
(mime-editor/insert-file-verbose): New function.
(mime-find-file-type): Bind `case-fold-search' as t.
(mime-editor/translate-header): Use `eword-encode-header' instead of
`mime/encode-message-header'.
(mime-editor/sign-pgp-elkins): Use `as-binary-process'.
(mime-editor/translate-single-part-tag): Use `eword-encode-header'
instead of `mime/encode-message-header'.
(mime-editor/normalize-body): Don't call `mime-encode-region' if
encoding is nil.