Upgrade to Vue 3 (#331)

* Dependency updates

* Upgrade to Vue 3 (WIP)

* Fix reactivity of imported domains (the object gets replaced by a proxy)

* Cleaner access of proxy for domain imports

* Remove upgrade note about reactivity issue

* Fix locale switching

* Remove upgrade note about language issue

* Patch Vue 3 issues with pretty-checkbox-vue

* Remove upgrade note about checkbox issue

* Fix extra slot for checkboxes and radios

* Set checkmark as default for extra slot

* Use patched pretty-checkbox-vue from do-vue

* Fix removing domains

* Switch to main do-vue/do-bulma