vm-list-mime-part-structure should better manage the buffer

[Uday Reddy]

vm-list-mime-part-structure displays the MIME part structure, typically in the window that the message presented in, and then it just stays there. The buffer has to be explicitly killed to get rid of the information.

It might be better to display the information in the minibuffer, perhaps, or an electric window like electric-help.

[Uday Reddy (reddyuday)]

Fix committed in rev. 1296