Split out source listings in gdk/, gtk/ and demos/gtk-demo/, and clean up build files

Hi,

As a part of the drive to make the generated sources not dist'ed (MR !1001 (closed)), this splits out the source listings from gdk/Makefile.am and gtk/Makefile.am and demos/gtk-demo/Makefile.am so that the source listings can be also consumed by the NMake Makefiles used to generate the sources, which will be used to generate the enumeration sources, that we need.

This also adds a mechanism in the afore-mentioned NMake Makefiles to re-generate demos\gtk-demo\demos.h.win32 to enable or disable the font features demo program.

This could also pave the way to cleans up the autotools build files a bit so that we will no longer need to generate the source listings for introspection during 'make dist', assuming that we don't run into the dreaded "command line too long" problem on Windows.

Note that this is not applicable to master since we are Meson-only there.

With blessings, thank you