(mime-view-define-keymap): Add new binding `mime-preview-show-header'

(mime-view-define-keymap): Add new binding `mime-preview-show-header'
for C-c C-v C-f and C-c C-v h; add new binding
`mime-preview-show-content' for C-c C-v C-c; add new binding
`mime-preview-hide-header' for C-c C-d C-f and C-c C-d h; add new
binding `mime-preview-hide-content' for C-c C-d C-c.
(mime-preview-toggle-display): New function.
(mime-preview-toggle-header): Add new optional argument
`force-visible'; use `mime-preview-toggle-display'.
(mime-preview-toggle-content): Likewise.
(mime-preview-show-header): New function.
(mime-preview-show-content): New function.
(mime-preview-hide-header): New function.
(mime-preview-hide-content): New function.