Rework The Visual Studio 2008 Build Process

This patch makes the build of G-I on Visual Studio 2008 into a two-step
process, as it would make it clearer and easier to the one building G-I by:

-Splitting up the property sheets into multiple parts, where pre-configured
 code (which is currently shipped with the tarball) is copied using custom
 build steps, so that they can be re-copied when they are updated.  This
 also removes the need for setting environment variables before launching
 the Visual Studio Project to build the G-I DLL, the tools written in C and
 the _giscanner Python2 module.  The Python2 paths (set to Python 2.7.x)
 are now set in a property sheet, which can be updated quite easily, if
 needed.
-Use a set of NMake Makefiles to build the introspection files for GLib,
 and also the .gir's that are shipped with G-I, and make it clear to people
 what things are needed to build the introspection files.
-Standardizing across the board that we now use $(ApiVersion) for the .lib
 files and the "installation" of headers, etc.

Similar updates to the Visual Studio 2010 files will follow later.