path improvements/fixes:

- replace more std::wstring with Native or VfsPath; wstring_from_utf8 ->
NativePathFromString
- replace sequences of Join(Path(), Basename+extension) with
ChangeExtension in wsdl, CacheLoader
- add Path::IsDirectory to replace .empty() / path_is_dir_sep(.back()).
also changed behavior to reflect the fact that "" is the VFS root
_directory_
- Path::Join now allows 2 identical path types (e.g. VfsPath) or one
char* literal (prevents inadvertently introducing non-safe characters).
to convert from wstring or wchar_t, use an explicit ctor (e.g.
VfsPath(wchar_t_string))

This was SVN commit r9091.

source/graphics/ColladaManager.cpp

@@ -248,7 +248,7 @@ VfsPath CColladaManager::GetLoadableFilename(const VfsPath& pathnameNoExtension,
 	const wchar_t* realDaePath = wcsstr(realDaeBuf, L"mods/");
 
 	// cachedPmdVfsPath is "cache/mods/whatever/art/meshes/whatever_{hash}.pmd"
-	VfsPath cachedPmdVfsPath = Path::Join(L"cache/", realDaePath);
+	VfsPath cachedPmdVfsPath = Path::Join("cache", VfsPath(realDaePath));
 	cachedPmdVfsPath = Path::ChangeExtension(cachedPmdVfsPath, extension);
 
 	// If it's not in the cache, we'll have to create it first