Enhance widget customization: shortcut to hide flag and add disabled state

[IguJl15]

I'm using the component and missed two main things:

• Boolean property to quickly show/hide the flag without the need to write a builder (similar to #156)
• Make onChanged callback nullable to turn the dropdown disabled

I think I will develop it and open a PR someday this week. Is this ok for you?

[caseyryan]

Hi! Yes, that's ok. Unfortunately I don't have much time to maintain my open source projects at the moment, so if you do it, I'd be grateful