WSUG: Convert the "User Interface" chapter to AsciiDoc.

Add the "asciidoc.conf" compatibility configuration file from
Asciidoctor. Although we don't use Asciidoctor it gives us macros that
keep us from losing some useful DocBook elements. Update various CMake
files to support multiple AsciiDoc configuration files.

Leave most of the content intact for now. Hopefully the other chapters
aren't as laden with markup.

Change-Id: Id69757342b86abb2b3130cb61e90f5695a26ea8a
Reviewed-on: https://code.wireshark.org/review/3680
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>