Add methods for minimizing and unminimizing the window/drawing

[sunjay]

Follow-up issue from #49.

It would be great if we could minimize and unminimize the window/drawing programmatically.

The set_minimized method on Window should come in handy for that.

• [ ] Add is_minimized, minimize, and unminimize methods to Drawing

This issue is partially blocked on new events becoming available: rust-windowing/winit#1578

• The minimize and unminimize methods can be added and stabilized with no issues
• The is_minimized method may be added, but should be marked unstable like is_maximized

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