Investigate suitability of zlib-ng to replace zlib

The zlib project has a fairly long history of ignoring pull requests on GitHub. The zlib-ng project is a more actively maintained alternative that aims to be API compatible. CMake is considering adding zlib-ng to the set of package names it considers in its FindZLIB module. Given those points, it is a valid question whether Qt should consider replacing the zlib it currently vendors in its qtbase/src/3rdparty area with zlib-ng instead. This should be investigated as a potential improvement to maintainability.

It has the same (liberal) license as the original, includes optimisations for modern systems and has unburdened itself of support for antique systems and compilers, enabling further optimisations.