Picking table does not get updated when Slices: Occupied is selected

[thorstenwagner]

I can at least reproduce this with filaments when picking tomograms.

If start picking on a new slice, this table gets not updated when "Slices: Occupied" is selected.

@mstabrin Could you have a look?

[thorstenwagner]

@mstabrin bumb - do you have time?