Convert Visual Studio build system to NMake Makefiles (gtkmm Master)

Hi,

As the Visual Studio projects are manually-maintained, and it is often the case that they get outdated because sources are often added or removed, this merge request converts the Visual Studio build system to NMake Makefiles so that it can also make use of the various filelist.am's which the autotools build use, so that additions and removal of sources will less likely break the Visual Studio builds.

This also updates gendef.cc a bit do work around the issue where some CRT functions are done in-line with Visual Studio 2015 and later, due to their use of UCRT, so that builds won't break during building the gtkmm DLL. Unfortunately there seems so far no other way to work around this issue.

Similar work will also be done for the upcoming gtkmm-3-24 branch as well.

This also makes things a bit more modular, so to ease maintenance of things.

With blessings, thank you!