Implement Script that can globalize a local flood-fill operation

[philippotto]

With #5733 it becomes possible to locally correct a segmentation merger by splitting a cell into two components. However, the floodfill operation is constrained to only work in a certain bounding box. If the floodfill operation is cancelled because the limiting bounding box was exceeded, the split is incomplete. In that case, a bounding box is created in WK automatically. That bounding box then contains information, such as the seed voxel, the segment id.

Given these bounding boxes and a dataset/volume annotation a script can "continue"/"expand" the flood fill operation to the entire dataset. In particular, the script should be able to consume multiple bounding boxes so that multiple flood fill operations can be globalized.