Lineage Registration: add a command to copy spot labels from one dataset to the other

[maarzt]

Similar as there is currently a copy tag set functionality in the lineage registration plugin, there should be a functionality for copying the spot label from one dataset to another.

There should be a Button added to the lineage registration plugin to copy spot labels from one dataset to the other.

Difficulty: labels are defined on spot level. But lineage registration works on a branch level. So copying can only happen on are branch level. But usually, for the datasets we work with. Usually all spots in a branch should have the same label.