Streamline syncing between components' state and Vuex

• Related discussion: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15333#note_213672404

While developing some frontend features, we sometimes need to reflect a component's state in a Vuex store. In the past, we've dealt with this by developing wrapper components that take care of keeping the component and the store in sync:

• gl-modal-vuex: this component wraps gl-modal and keeps its visibility in sync with a store module
• gl-toggle-vuex: this one wraps gl-toggle in order to sync the toggle's state with the store

This raises a few questions:

• Should GitLab UI components have support for syncing their state with Vuex out of the box?
• If the answer is yes, how should we implement this support?
  • Should it be by providing wrapper components? This would basically mean that we would need to move gl-modal-vuex and gl-toggle-vuex to GitLab UI
  • Or should our components support this in their base implementation?
  • In any case, are we able to abstract this pattern enough that we don't have to reimplement it for each component that needs it (ie move the logic to a mixin)?