Update the project configs to build GDK/GSK as a static lib and include
them into the GTK+ DLL as a monolithic DLL, which is in line with what is
done in the autotools builds, since the code changes needed for Windows
builds for a monolithic build are now in place.
We now need C99 features from the compiler which are only supported by
Visual Studio 2013 and later, so drop the MSVC 2008~2012 projects, and make
the baseline supported Visual Studio version be 2013. Update the build files
as a result.
