UX issue: when I switch data in the explorer

[sidiDev]

UX issue: when I switch data, sometimes I thing there’s no data and the data appears after some seconds, will be better to have a spinner or loader that appears before the data shows up in the page, this will help improve the user experience otherwise it’s confusing.

Oh, I just noticed "Loaded..." that appears at the top, but that’s invisible, and I'm sure 99% of users may not realize it.

We want a more visible loading indicator when we switch object types in the data explorer.

[jackfischer]

Related to https://github.com/edgedb/edgedb-ui/issues/190