Follow-up: Enable volume Interpolation with depth > 2 and clarify usability

[philippotto]

Detailed Description

The interpolation itself already supports higher depths, but the usability might need some tuning. We should clarify the following questions:

• Is a depth of 4 meant as the exact depth or as the maximum depth? In other words: what happens if the depth is set to 4, but I only advance by 3 slices and brush. Will this
  • use the volume data from 4 slices ago for interpolation (which is likely interpolated itself and therefore not a good source)
  • use the volume data from 3 slices ago for interpolation
  • don't do any interpolation?
• In general, can/should we let the user know from which slice the next brush action would use data for the interpolation?
• Can we let the user know which slices where drawn by interpolation so that proof-reading of interpolated slices is easy to do after annotating a bunch of slices?

Context

See #6162