Only generate VC'10 .filters file when project has subfolders

--HG--
extra : rebase_source : 376901c3380255ccfab115a3b2a3bcfac90364f1
Split more of VC 200x exporter into function call-outs

--HG--
extra : rebase_source : e84783a14ed6069c889b46526666a5be476b85a0
Merge https://github.com/annulen/premake