Dashboard avoid reliance on broadcast emits

[stacey-gammon]

Currently dashboard relies on a signal 'ready:vis' being emitted from root scope from the visualizations. That emit causes issues with visualizations embedded in tooltips, but taking it off rootscope broke dashboard (#14674).

Dashboard and embeddables should be able to be written in any framework so we need to break this reliance and put it back on the embeddable API, to give a way for the embeddables to communicate to dashboard that they have finished rendering.

Why do we even need this knowledge? Well currently, it's used to re-fetch a request in courier to trigger a fetch on saved searches (hence the bug). Other than that, I believe it's useful for plugins to know when the dashboard is completely finished loading.

[timroes]

I am going as far and assume with the removal of Angular (#115464) this is no longer a problem and closing it.