LibGUI: Use parrent window icon for MessageBox dialog icon

Author: https://github.com/asliturk
Commit: https://github.com/SerenityOS/serenity/commit/c6944f8cc21
Pull-request: https://github.com/SerenityOS/serenity/pull/1756
Reviewed-by: https://github.com/awesomekling

Libraries/LibGUI/MessageBox.cpp

@@ -36,8 +36,10 @@ namespace GUI {
 int MessageBox::show(const StringView& text, const StringView& title, Type type, InputType input_type, Window* parent_window)
 {
     auto box = MessageBox::construct(text, title, type, input_type);
-    if (parent_window)
+    if (parent_window) {
         parent_window->add_child(box);
+        box->set_icon(parent_window->icon());
+    }
     return box->exec();
 }