MSVC Builds: Fix _giscanner projects

The scanner[lexer|parser] sources are now in giscanner/, not $(srcroot), so we need to update the Visual Studio project files to make sure things build properly.

There are also some updates in the gi-introspection-msvc.mak NMake Makefiles so that the generated .gir's conform better to the ones produced by the autotools builds, and GObject-2.0.gir has the correct header specified.