Since the status of the GDK broadway backend is more or less unsupported,
drop the projects that build gtk4-broadwayd and gdk-broadway, and update
the projects to not to refer to them.
However, keep the Broadway configs for now as we will later transform
them to become configs for Vulkan, so bascially besides "installation"
parts and output settings, they will do the same as their Release|Debug
counterparts with no support for Broadway.
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.
