This adds a PlatformToolset tag in the project configs so that we can
provide support for Visual Studio 2012 with relative ease as the format
of the VS 2012 projects are only slightly different from their VS 2010
counterparts.
We can then use a script like the one used in GLib[1] to copy the VS2010
projects and replace the necessary tags to create the VS2012 projects.
This also cleans up the projects and property sheets, as there were some
unwanted/unneeded entries in them.
[1]: http://git.gnome.org/browse/glib/commit/?id=76cecf061b377d30e5422cdddb1fb9d19c52421d
-Reinstate build/win32/vs10/gtk+.sln with the correct EOL (DOS/Windows), so
that it will be correctly recognized by Windows instead of having the
annoying "Unrecognized Visual Studio Version".
-Update property sheets to reflect on new headers added
-Change the demo program to be gtk3-demo.exe, to be consistent with the
names on other platforms, and updated/renamed related project/solution
files, and added overlay.c to the list of demo sources.
