Restore Vol-E settings when returning to previous volumes ("memory")

## Use Case

Requested by @carolinecch! Users previously requested the ability to keep the same intensity settings when switching between volumes (#242).

Caroline also described a scenario where she wants to view two different images with different settings and switch quickly between them:
1. User views Volume 1, which appears with best fit contrast settings, and dials in settings.
2. User opens Volume 2, which appears with best fit contrast settings, and dials in settings.
3. User opens Volume 1 again.
a. In the current behavior, Volume 1 is reset back to the best fit contrast settings.
b. Caroline would instead like Volume 1 to keep the previously-used contrast settings.

## Solution

- [ ] Create a component that stores a dictionary mapping from source to viewer settings in CFE, which updates whenever viewer settings change.
- [ ] When the source image changes and the user goes to a previously-loaded volume, load the previous viewer state again.
- [ ] Obey other previously-set behaviors (e.g. #242 keep channel intensity values)

## Alternatives

- What settings should we drop or ignore? Should we only keep channel intensity settings? Colors?