Fixed a bug that restoring files doesn't work due to missing parent directory

The root cause was path.Dir can't handle Windows paths that use \ as the
separator.

src/duplicacy_utils_windows.go

@@ -126,3 +126,8 @@ func joinPath(components ...string) string {
 	}
 	return combinedPath
 }
+
+func SplitDir(fullPath string) (dir string, file string) {
+	i := strings.LastIndex(fullPath, "\\")
+	return fullPath[:i+1], fullPath[i+1:]
+}