Upstream Checkbox.vue to vue lib

[nickvergessen]

https://github.com/nextcloud/activity/blob/master/src/components/Checkbox.vue should be upstreamed (https://github.com/nextcloud-libraries/nextcloud-vue) to the NcCheckboxRadioSwitch component.

There was no time to do it adhoc, relevant changes:

• It was round
• it was also to big
• didn't had a good enough focus effect for a such small component