RFC: Add support for tnef / winmail.dat

[butrus]

Expected behaviour

Engrampa can open (and perhaps also edit and/or create) TNEF archives also known as winmail.dat, used by windows to send email-attachements.

Actual behaviour

TNEF seems not to be supported even in the git-version.

Note: TNEF parser is already implemented in the tnef and ytnef-tools packages (as it is called in debian and co.).

[butrus]

Additional notice: Actually there is the libytnef c-library which can be directly used and is a part of the usual linux distros.