This makes the rest of the sources that are generated being put in GDK's build
directory for the Visual Studio projects, instead of being placed in GDK's
source subdir, so that:
* We can avoid polluting GDK's source tree.
* The Visual Studio builds won't interfere with the Meson builds.
Update the project files to look for the generated sources and headers in the
right places, as well as making the introspection builds look for the generated
sources as well.
Also remove the gtk3-gen-srcs property sheets and put the relevant parts in the
corresponding gtk3-build-defines property sheets, to clean things up a bit.
It turns out that we must put Directory.Build.props in win32/vs10 and let
automake copy it over to win32/vs15/... so do that, and only dist the copy
in win32/vs15.
Update the NMake Makefiles used for generating the various sources be able to
cope with an ARM64 build environment, and update the project files, in
particular for Visual Studio 2017 (VisualStudioVersion 15.0) so that we can
pull in the system .lib's that were somehow excluded from the default list
on ARM64 builds.
Also, add a custom Directory.Build.props in win32/vs15 so that we do not
try to build with the Windows 8.1 SDK by defualt, which is not ready for ARM64
builds, but instead uses the appropriate Windows 10 SDK that supports this.
Update the README.win32 file to give people instructions on how ARM64 builds
can be carried out.
