Custom container component for programmatically opened modals

[dword-design]

Description

Allow to instantiate a <b-modal-orchestrator /> component to control where programmatically opened modals are rendered.

Why Buefy need this feature

When opening modals programmatically, the modal is currently opened in a fixed root-level div. But e.g. when having a sidebar layout, you want to show the modals in the main part of the layout, not above the sidebar.

bootstrap-vue-next requires to instantiate a <b-modal-orchestrator /> component that is then used as the root. I think they are using a global state object to then render the modals in the orchestrator. This way we also wouldn't have to mount a new Vue instance.