[VM] Point-to-point attachment operations

[Uday Reddy]

Spurred by a suggestion from 'emacs user', I have now added a number of 
"point-to-point" operations for attaching objects to message 
compositions.  I think they make a nice addition to VM's functionality.

They are in the trunk now, and will be part of the next release.

Cheers,
Uday


Point-to-point attachment operations
....................................

A number of "point-to-point" operations allow you to attach objects
from other editing contexts to a message you are composing.

   You can visit a directory in Emacs (*note Dired: (emacs)Dired.), and
run `vm-dired-attach-file' on any file.  The file be attached to your
message composition.  You can also mark a set of files in Dired and run
`vm-dired-do-attach-files' to attach all of them.

   You can use your Window system to drag and drop a file into a
composition buffer (`vm-dnd-attach-file').

   When you visit a folder in VM, you can attach a message from the
folder by running `vm-attach-message-to-composition'.  When viewing a
message that has MIME attachments, you can attach any of those
attachments to your message composition by using the `$ a'
(`vm-attach-object-to-composition') function.  (*Note Operating on MIME
attachments::.)  This operation is also available on the pop-menu for
attachments.

   In all these cases, you will be prompted for the message composition
buffer to which you would like to attach the objects.  The default is
the latest message you have been composing, as indicated by the Emacs
buffer ring.