This improves the source generating process by:
* Generating the sources in the libgtk build directory, so that we do not
pollute the source tree.
* Support generating gtk[private]typebuiltins.[c|h] in the Visual Studio
projects by using NMake internally.
* Update the projects and the introspection build files so that the generated
sources and headers can be found.
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.
