Don't add VCTargetsPath nodes for VS2012.

Current version of VS2012 doesn't create them, it was only done by some
interim version.  And the presence of this code causes VS2013 Preview to
freeze, see http://support.microsoft.com/kb/2858347/en-us
VS201x: *.filters files should begin with UTF-8 BOM too.
Add support for Visual Studio 2013 Preview.
VS201x: list all files in .vcxproj.filters files.

Natively saved filter files don't list just the filter groups, but
enumerate all files and file them into their respective groups
explicitly.  Do the same.
Merge branch 'vs2013'