Provide a way to distinguish unparsed from successful

[CohenCyril]

We need to distinguish

1. not yet parsed. (e.g. for long files and slow code)
2. parsed and successful
3. parsed and failed.

Right now 1., 2. print the same way, while 3. is underlined in red.

We could even add
4. parsing in progress

We could have color code (configurable by the user) to distinguish between 1. 2. and 4.
By default we could

• highlight in blue/green successful evaluations, (this would look like manual mode and be less destabilizing for new users of the continuous mode)
• and/or overline in grey what is not yet processed at all (e.g. the end of the file)
• and optionally overline in orange things that are under evaluation, (so we can witness visually something is taking time).

CC @rtetley
Cf https://coq.zulipchat.com/#narrow/stream/237662-VsCoq-devs-.26-users/topic/How.20to.20know.20what.20coq.20did.20do.20and.20what.20it.20did.20not.20do.3F