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/gui/scripting/ScriptFunctions.cpp

@@ -433,7 +433,7 @@ void ForceGC(void* cbdata)
 
 void DumpSimState(void* UNUSED(cbdata))
 {
-	NativePath path = Path::Join(psLogDir(), L"sim_dump.txt");
+	NativePath path = Path::Join(psLogDir(), "sim_dump.txt");
 	std::ofstream file (StringFromNativePath(path).c_str(), std::ofstream::out | std::ofstream::trunc);
 	g_Game->GetSimulation2()->DumpDebugState(file);
 }