Use new NSString lisp methods

Use new NSString lisp methods

* src/nsfont.m (ns_otf_to_script):
(ns_registry_to_script):
(ns_get_req_script): Use NSString conversion methods.
* src/nsimage.m ([EmacsImage allocInitFromFile:]): Use NSString
conversion methods.
* src/nsmenu.m (ns_menu_show): Use NSString conversion methods.
* src/nsselect.m (symbol_to_nsstring):
(ns_string_to_pasteboard_internal): Use NSString conversion methods.
* src/nsterm.m (ns_term_init):
([EmacsView initFrameFromEmacs:]): Use NSString conversion methods.
* src/nsxwidget.m (nsxwidget_webkit_uri):
(nsxwidget_webkit_title):
(js_to_lisp): Use NSString conversion methods.
(build_string_with_nsstr): Functionality replaced by NSString
extensions.