Feature request: ability to copy a po_message

[F Wolff]

We are trying to use libgettextpo from Python, and hitting an issue not
being able to duplicate (copy) a po_message.

While it is probably possible to go through all members of a unit to try
to imitate this, it will be hard to maintain and bug prone. This request
is for a utility function in libgettextpo to make a deep copy of a
po_message, in order to enable us to deepcopy a message from one file
into another, or simply to duplicate a message for some other purpose.
This way we will be able to free files without the risk of attempting to
free memory twice since a shallow copy was made.

If I missed this functionality, any help in doing it properly would be
appreciated. 

Thanks!
Friedel