Do not issue backend RPCs for data explorer instances that are in the background

[wesm]

If a user opens a data explorer (DE) window, and then switches back to a code editor, hiding the DE, any changes that trigger data or schema updates will continue to issue backend requests from the DE component. If there are many DE windows open, this could result in burdensome amounts of computations that are never actually seen by the user.

I suggest that we should defer such computations until the window is reactivated and the UI starts painting again. Hopefully the business logic for doing this isn't too onerous but it seems like it would be for the best