Surface: new Extend icon for existing Surface_ExtendFace command

The icon is added to the `.qrc` resource file.

The command is added to the Surface toolbar.

src/Mod/Surface/Gui/Workbench.cpp

@@ -71,7 +71,8 @@ Gui::ToolBarItem* Workbench::setupToolBars() const
     surface->setCommand( "Surface" );
     *surface << "Surface_Filling"
              << "Surface_GeomFillSurface"
-             << "Surface_Sections";
+             << "Surface_Sections"
+             << "Surface_ExtendFace";
 /*  *surface << "Surface_Cut"; */
 
     return root;