Add custom icon for checkbox component

[tux2nicolae]

We use MaterialIcons in our app instead of MaterialComunityIcons and we have a problem with the checkbox not having a property for specifying a custom icon

p.s Sorry for opening again, I've rebased the branch and it seems that in the process the old PR was automatically closed

[callstack-bot]

Hey @tux2nicolae, thank you for your pull request 🤗. The documentation from this branch can be viewed here.