revert on_trait_change to observe changes in shell editor

[aaronayres35]

closes #1612 and closes #1620

This PR simply takes the easy way out of reverting the on_trait_change to observe changes made to ShellEditor in #1523

I aim to open a separate PR that fixes the use of observe. This PR should be seen as a back up plan to avoid the issues in case there is trouble coming to the real solution and we don't want to hold up the release.
Note the migration from on_trait_change is not yet complete and won't be by the upcoming release, so it is not as though this will be the only use of on_trait_change in the codebase if this PR does get merged.

In any case I plan to try to actually fix the issue in a separate PR in which case this can simply be closed.

[rahulporuri]

LGTM. This works for us for now. We've wanted to make the on trait change ->observe changes progressively so we can defer this one to a later time when we're not trying to push out a release.