Enable or disable resizing the window

[sunjay]

Follow-up issue from #49.

It would be great if we could disable resizing the window as some drawings only work with certain window sizes. The set_size and maximize methods would still work, but the window would otherwise not be resizable by the user.

This requires being able to change that property of the window dynamically. The set_resizable method on Window should come in handy for that.

• Add is_resizeable and set_resizeable methods to Drawing

If you would like to work on this, please leave a comment. I am happy to provide mentoring instructions.