Mixin for checkbox border and radius

[grudra7714]

Added mixins for checkbox container border and radius Demo

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

:memo: Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If your company signed a CLA, they designated a Point of Contact who decides which employees are authorized to participate. You may need to contact the Point of Contact for your company and ask to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the project maintainer to go/cla#troubleshoot.
• In order to pass this check, please resolve this problem and have the pull request author add another comment and the bot will run again.

[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?