Mixin for checkbox border and radius

[grudra7714]

Added mixins for checkbox container border and radius
Demo

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[grudra7714]

I signed it!

[googlebot]

CLAs look good, thanks!

[kompike]

Fixes #182, fixes #184.

[kompike]

I'm very interested in changes provided by current PR. I would like to know if there are any timelines or estimates for processing this code, or at the moment I should create custom component with specified functionality for my project?
Thank you in advance!

[jouni]

If you need a checkbox that you can style freely, try vaadin-checkbox: https://github.com/vaadin/vaadin-checkbox

There’s a material theme available for it as well: https://cdn.vaadin.com/vaadin-material-theme/0.3.0/demo/checkboxes.html

[gerarts]

CI failure seems to be an issue with the CI server, not the PR. Can this be merged?