Like what was done in the *.vcxprojin templates, consolidate project configs
that are repetitive, and use the 'Condition=' directive to distinguish between
configuration and/or target platform differences.
...for Visual Studio 2010 and later, since we used to copy them and
merely update the value of $(VSVer) during 'make dist' as needed.
Instead of doing that, move the property sheets for Visual Studio 2010,
which is the base version that we use, into a directory of its own, and
make the Visual Studio 2010~2022 projects look for them in that
directory, and set VSVer according to the toolset version identified by
each of the project files.
The ARM64-specific property sheet needed for VS2017 and the VS2008
property sheets will remain as they are.
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.
