Existing annotations are overwritten on interpolation

[bauerdavid]

[from a bug mentioned in #20 by @jo-mueller]
Describe the bug
When interpolating a given label value, all other label slices are overwritten (if the masks do not overlap, with zeros).

To Reproduce
Steps to reproduce the behavior:

1. Create a 3D Labels layer
2. Create some mask with 1 values (use multiple slices, you can use the Interpolation widget)
3. Annotate two distinct slices with 2 labels. The slice interval between the annotations should overlap with that of the 1 labels.
4. Click the Interpolate button
5. See the results

Expected behavior
If the interpolated labels in the slices do not overlap with existing masks, then leave them alone. Otherwise overwrite them with the new label value.

Additional context
I'm not sure, but maybe the user could select, whether the interpolated slices should overwrite existing labels.

[bauerdavid]

resolved in de5ad19