CodeChunk and CodeExpression: indicate execution status in outputs and elsewhere

[nokome]

Currently, the only place the user has an indication of whether a CodeChunk or CodeExpression is currently running is the spinner icon:

This is OK for code chunks with less than 30 lines, but I have found that when editing longer code chunks, I will often make changes at the end of the code chunk, press Shift+Enter and then have to scroll up to check that the chunk is actually running. I'd prefer to have behavior like in RStudio where the output fades out, it shows a spinner and a green line to the side indicating that the code is currently being executed, and the new output fades in:

[nokome]

Bumping this issue, as I think this would still be a useful improvement.

In addition to fading out outputs on executeStatus == Running, i think we should do it for executeRequired == SemanticsChanged | DependenciesChanged | DependenciesFailed as an additional visual cue that the current outputs are stale.