Navigation in mime/viewer-mode
******************************

   `mime/viewer-mode' has following functions:

`<u>'
     goes to the upper content (returns to the Summary mode if the
     cursor is sitting on the top content (*1))

`<p>'
     goes to the previous content

`<M-TAB>'
     goes to the previous content

`<n>'
     goes to the next content

`<TAB>'
     goes to the next content

`<SPC>'
     scrolls up

`<M-SPC>'
     scrolls down

`<DEL>'
     scrolls down

`<RET>'
     goes to the next line

`<M-RET>'
     goes to the previous line

`<<>'
     goes to the beginning of message

`<>>'
     goes to the end of message

`<v>'
     playbacks a part (*2)

`<e>'
     extracts a file from a part (*2)

`<C-c C-p>'
     prints a part (*2)

`<f>'
     displays X-Face in the message

`<mouse-button-2>'
     drives mouse button in preview-buffer.

     For content-button, it playbacks a part (*2)

     For URL-button, it drives WWW browser

*[Notice]*
     (*1) Not return to the Summary mode unless tm-view has been setup
     using tm-mh-e, tm-vm, gnus-mime, tm-gnus, tm-rmail etc.

     (*2) Actual playback/extract/print will be performed by a method.