Right click on button
Thank you for providing such a wonderful library.

Can I add right click event on button?

I'm making mine sweeper for my hoby, and I'd like to add "Flag" function to right click.
![image](https://github.com/simonkrauter/NiGui/assets/150841863/f2a8d3c5-5288-4c6b-91c6-1fb28fa8add7)
