By doing so, we can drop the autotools rules that we use to generate the
file lists and rules for building introspection via NMake Makefiles for
Visual Studio builds via the project files.
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.
This way, we can have the introspection NMake Makefiles share more items
with the source generating bits.
Update the Visual Studio projects accordingly
Currently, introspection is not well-supported for ARM64 Windows builds as:
* There is no official Python release for ARM64 Windows, but it is currently
possible to build it with sufficient support for G-I, however...
* The tooling in Python still needs to be updated to enable ARM64 Windows
builds for use with g-ir-scanner and friends, and...
* Introspection builds must be done on an ARM64 system, since we are running a
dumper binary to generate the .gir files
This will attempt to prepare things for building GTK's introspection files on
Windows ARM64, but will require a custom installation of Python as noted above,
for the time being
