Bug 736720: Fix ustring for Visual Studio Builds

Update the property sheets so that SIZEOF_WCHAR_T is defined as 2 when
the glibmm library is built.  This ensures that the correct code path in
the ustring class is taken for Visual Studio builds, which avoids an
unhandled GLib::ConvertError exception.

	* MSVC_Net2005/glibmm-build-defines.vsprops:
	* MSVC_Net2008/glibmm-build-defines.vsprops:
	* MSVC_Net2010/glibmm-build-defines.props: Define SIZEOF_WCHAR_T
	  when building glibmm so that ustring will use the correct code
	  path on Visual Studio builds.