WSUG: Convert the ``File I/O'' chapter to AsciiDoc.

Leave most of the content intact for now. Remove images for
no-longer-supported versions of GTK+. Add an example for building the
Guides to README.cmake.

Change-Id: Id9e6a308c91b594d1fb7f107d7b9b28074a92a8b
Reviewed-on: https://code.wireshark.org/review/3931
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>