(optionally) display cell execution status

[nthiery]

Elevator Pitch

Decorate cell inputs and outputs on the left with an indicator of the execution status, like in the Notebook viewer:

[2]   1 + 1
[2]   2
[*]   1 + 3
[ ]   1 + 4 

Motivation

Not showing the execution status is nice and lean. However, there are many occasions where executions are not immediate; as a speaker it's then stressful not to see at once whether one fumbled a shift-Enter, or the execution is taking some time, or the kernel is having trouble. Also, the indicator helps both speakers and readers distinguish between code and text cells.

Design Ideas

Some people could possibly prefer the lean version without indicators. If this is the case, maybe this could be configurable.