quartz: Implement gtk_clipboard_get_selection

This function is trivial and can be copied from the X11 implementation.

https://bugzilla.gnome.org/show_bug.cgi?id=772281

gtk/gtkclipboard-quartz.c

@@ -1239,3 +1239,17 @@ _gtk_clipboard_store_all (void)
       list = list->next;
     }
 }
+
+/**
+ * gtk_clipboard_get_selection:
+ * @clipboard:
+ *
+ * Since: 3.22
+ */
+GdkAtom
+gtk_clipboard_get_selection (GtkClipboard *clipboard)
+{
+  g_return_val_if_fail (GTK_IS_CLIPBOARD (clipboard), GDK_NONE);
+
+  return clipboard->selection;
+}