0029591: Improvements in the class BRepOffset_Tool

0029591: Improvements in the class BRepOffset_Tool

- Add the flag ExtensionMode in the method BRepOffset_Tool::EnLargeFace, defining the mode of extension of the surface of the face. Old behavior is to be remained the default one.

- Fix wrong building of extended face on a closed surface. Now, if the face is closed in U direction (like cylinder) but the seam edge is shifted from 0 position, the resulting extended face has properly connected seam edge.

- Add new public static method BRepTools::DetectClosedness(), which checks whether a face is closed in U and V directions.