Visual Studio projects: Simplify building font features demo

Chun-wei Fan requested to merge improve-font-features-msvc into gtk-3-24


From the commit message:

Improve the NMake Makefiles that is used to re-generate demos.h(.win32) by re-generating the gtk3-demo Visual Studio projects, so that one would no longer need to do that manually.

Update the build instructions accordingly.

With blessings, thank you!

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