Make NS toolbar use NSString instead of C strings

Make NS toolbar use NSString instead of C strings

* src/nsfns.m ([NSString stringWithLispString:]): Ensure that the lisp
object is actually a string.
* src/nsmenu.m (update_frame_tool_bar): Convert to NSString instead of
C strings.
([EmacsToolbar addDisplayItemWithImage:idx:tag:labelText:helpText:enabled:]):
No need to convert to NSString here anymore.