Since demos.h is now generated according to the platform for which GTK+ is
built, don't distribute it. Generate a Windows-specific demos.h.win32 and
distribute that instead, in which the Visual Studio build files will copy
it to demos.h, so that the build will proceed normally.
https://bugzilla.gnome.org/show_bug.cgi?id=749622
Add a utility project to get config.h and gdkconfig.h from their *.h.win32
(or win32_broadway, if applicable) counterparts, using custom build rules,
so that these "generated" files can also be removed on clean and
"regenerated" upon update. This also enables the removal of configs in
certain projects that isn't really needed as a result.
Also update and merge the projects and property sheets to include a single
property sheet that it needs, which will then in turn include the other
property sheets that is needed, so that things are cleaner.
Updates to the Visual Studio 2010 projects will follow later.
