Update NEWS: Add S7 Communication dissector and the extcap interface.

Also add the S7 Communication dissector's author to AUTHORS.
Mention that the Qt UI is now the default.

Change-Id: Ie2629333fd48bbe1ce95052292336a4f8608ea17
Reviewed-on: https://code.wireshark.org/review/3988
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Reviewed-by: Michael Mann <mmann78@netscape.net>