Use more efficient implementation of merge-surfaces DeleteCellsNearLabelBoundary

[schuhschuh]

For each connected label boundary, find closest points on surface. Use region growing on surface from which cells should be deleted while pruning when distance to label boundary exceeds the threshold. Keep track of already visited points. This should avoid a lot of the unnecessary distance calculations in vtkDistancePolyDataFilter. Moreover, as signed distance not needed, it would be best not to use this filter which also computes surface normals and dot products.

See also #744.