Convert Visual Studio build system to NMake Makefiles (gtkmm-3-24)

Hi,

As the Visual Studio projects are manually maintained in terms of the source file lists, it is often the case that they go out-of-date as sources are often added or removed. To remedy this issue and so to ease future maintenance, this adds a set of NMake Makefiles which can be used to build gdkmm and gtkmm (and the gtkmm demo program for Visual Studio 2015 or later, since some of the demo code use C++-11 features that are only supported in Visual Studio 2015 or later).

One question though, since similar work has been proposed as PR's/MR's for the C++-17'fied branches of libsigc++, glibmm, atkmm, and pangomm, will there still be future C++-11 releases of those libraries? If it is the case for these libraries, please let me know, so that similar work can be done for those branches as well.

With blessings, thank you!