Don't show focus ring on wxListBox

In native OS X apps, focus ring is not shown around NSTableView controls
(focus is indicated differently, with selection's color).

See eece498 for the same fix in wxDataViewCtrl.

src/osx/cocoa/listbox.mm

@@ -648,6 +648,7 @@ wxWidgetImplType* wxWidgetImpl::CreateListBox( wxWindowMac* wxpeer,
 
     wxNSTableView* tableview = [[wxNSTableView alloc] init];
     [tableview setDelegate:tableview];
+    [tableview setFocusRingType:NSFocusRingTypeNone];
     // only one multi-select mode available
     if ( (style & wxLB_EXTENDED) || (style & wxLB_MULTIPLE) )
         [tableview setAllowsMultipleSelection:YES];