0031547: Visualization - V3d_View creates V3d_Trihedron instance

0031547: Visualization - V3d_View creates V3d_Trihedron instance

- moved creating of V3d_View::myTrihedron from constructor to first attempt of displaying or calling it;
- moved creating of V3d_Viewer::myRGrid and V3d_Viewer::myCGrid to to first attempt of displaying or calling them;
- renamed V3d_Viewer::IsActive() to V3d_Viewer::IsGridActive();
- redefined V3d_View::Trihedron() and V3d_Viewer::Grid() by adding new argument toCreate
for opportunity to check the existence of the objects without their recreating.