Update and Overhaul Visual Studio 2008 Projects

	* MSVC_Net2008/gtkmm-build-defines.vsprops:
	* MSVC_Net2008/gtkmm-version-paths.vsprops: Added property sheets
	  to group commonly-used items in the projects, so that projects
	  can refer to them.  This will greatly simplify maintenance of the
	  project files.
	* MSVC_Net2008/gendef/gendef.vcproj:
	* MSVC_Net2008/gdkmm/gdkmm.vcproj:
	* MSVC_Net2008/gtkmm/gtkmm.vcproj: Move to MSVC_Net2008/, and clean
	  up by using the property sheets.  Add the newly-added sources as
	  needed, and adjust file paths accordingly.  Generate debug info
	  files for Release builds too, and remove the /vd2 build option,
	  which is a source of wierd crashes.  Do not build blank.cpp, as
	  it is not needed anymore.
	* MSVC_Net2008/demos/gtk-demo/gtk-demo.vcproj: Rename as
	  MSVC_Net2008/gtkmm3-demo.vcproj, and update file paths
	  accordingly.  Cleanup using the property sheets, and remove the
	  /vd2 build options, which is a source of crashes for MSVC 2008
	  builds.  Add the newly-added demo sources as well, and stop
	  building blank.cpp, as it is unnecessary.
	* MSVC_Net2008/gtkmm.sln:
	* MSVC_Net2008/filelist.am: Update file paths accordingly.