Fix toolbar item validation in the NS port

* src/nsterm.m ([EmacsView validateToolbarItem:]): Implement this
NSToolbarItemValidation method, which is called by AppKit when it
needs to validate an image item in a toolbar.  This solves, in
particular, the problem with having the "Save" icon on the tool
bar enabled at Emacs startup.  (Bug#62234)

src/nsterm.m

@@ -8581,6 +8581,10 @@ - (instancetype)toolbarClicked: (id)item
   return self;
 }
 
+- (BOOL) validateToolbarItem: (NSToolbarItem *) toolbarItem
+{
+  return [toolbarItem isEnabled];
+}
 
 - (instancetype)toggleToolbar: (id)sender
 {