Add the Visual Studio 2010 projects to build the GDK Broadway backend, just
like the Visual Studio 2008 project files in the last commit. Similarly,
split up the property sheets so that they are easier to maintain and can
be made more flexible for different build types. Also remove some unneeded
stuff from some of these items.
Also, fix the filter file completion for GTK, as a source file was excluded
for that and this was overlooked as it seemingly did not cause any trouble.
