Added custom iconed checkbox that can control its active, inactive and tristae icon

[Siqlain-Hanif]

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To Do

• Read contributing guide
• Check the original issue to confirm it is fully satisfied
• Add solution description to help guide reviewers
• Add unit test to verify new or fixed behaviour
• If apply, add documentation to code properties and package readme

[deandreamatias]

Hi @Siqlain-Hanif ! Thanks for your contribution!
I don't know if we can add this huge change.
I think that is create a funtionality for form field that does not add much worth and will be another thing to mantain.
If use a custom checkbox control can't be more simple, I think that solutions is that developers create your own form field with a custom control icon.

Sorry for this bad news, I thought that this feature will be more simple

[Siqlain-Hanif]

This was huge change and main reason was that flutter checkbox uses painter to render the icon so extension to that is not possible for this use case. So i had to come up with the new widget that uses the same API as flutter checkbox but render icons differently so other default icons can be used.

I agree this will be one more thing to maintain.

But any way i had a need of this in my project, so all is good and also i will publish this as a package so other can use this as well, In case they have similar requirements.

[deandreamatias]

Thanks for understanding!
When you implement this, maybe can add some example in readme using your package.
So, with this conclusion, I will close this PR