fix: Prefer `ref` over `id` in tooltip component

The id attribute needs to be unique in the whole document. If used for linking tooltips to their elements, it likely makes the parent component non-reusable. Therefore we should prefer ref which only needs to be unique in the current component.

The example code is take from https://github.com/bootstrap-vue/bootstrap-vue/blob/v2.0.0-rc.11/src/components/tooltip/README.md#b-tooltip-component-usage.