Prefetch/cache controls

## Use Case

As a scientist, I want to dial in the time series I want to see, allow the viewer as much time as it needs to load, then play through the data at high speed.

Based on suggestions from @Leigh-Harris and @antoineborensztejn

## Solution

- Expose UI for controlling prefetch behavior. Could be general "prefetch horizon/priority/memory limit" controls, or could be specific "queries" submitted by the user (e.g. "show me z slice 10 from time 10 to 300").
- Ideally, show load progress somehow. Either as a loaded region in the time slider, or a progress bar associated with a query (if we use that metaphor).

## Alternatives

Render movie or set of frames, then either export them or play back in the viewer? (Should have a separate issue for video export regardless)