Lightbox refactor
Refactor/rewrite the lightbox panel - should encompass (or make obsolete) #54, #39 (closed), and #38 (closed).
- Combine zoom and slice spacing settings into one option called slice spacing.
- The displayed slices are always scaled to fit the available display space.
- The number of rows/columns is automatically calculated to maximise use of the available display space (or perhaps can be overridden by an advanced setting).