1423584 - Have consistent and separate state for "loading" and "empty" panels

Status: NEW

(DevTools :: Netmonitor, enhancement, P3)

Description

[Nicolas Chevobbe [:nchevobbe]]

At the moment, some request detail panels (headers, cookies, response, …) have an empty state, which is used when we don't have data for them (yet).

First thing would be to have the same kind of empty state for all the panels. 
Second step would be to differentiate the loading state (we don't have data yet), from the empty state (this request does not have this kind of data)

I think this is important because you can sometimes see a flicker of the panel where it says "No data" and then the data appears. It can feel broken for the user.

For the loading state, either a simple "Loading..." text or an animated skeleton would be good (see https://codepen.io/oslego/pen/XdvWmd)