Rework The Visual Studio 2010 Build Process

This patch makes the build of G-I on Visual Studio 2010 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.

-Directing people to use the set of NMake Makefiles to build the
 introspection files for GLib, and also the .gir's that are shipped with
 G-I, which was added in a previous commit, as it would make it clearer
 what are needed to build the introspection files.
Visual Studio 2008 Build: Further Refinements

Fix up more grammar stuff in the README.txt file, and fix the CLEANFILES
line.