Filter for tunnel filling

Introduce a new filter able to fill tunnels. Still some robustness improvement to come. Also, I do not think the current name suit the filter, imo it only cover one of the multiple uses.

The filter remove a selected area (eventually) and then fill any non-watershed part of the mesh. It can fill a tunnel with the right selection, but it may also remesh into a "flat area" any selection.

@tiffany.chhim any though about the right naming ?