Path.Area: fixed non-circular curve handling, etc.

* Fixed non-circular curve orientation handling

* Section changed to use Part::CrossSection, because it seems
BRepAlgoAPI_Section has trouble with non-circular curves (LastParameter
becomes huge which causes discretization to produce many many points)

* Exposed Area.makeSections() to section with variable heights

* Modified Area.setPlane() to accept non-planar shape

* Exposed Area.getPlane() to obtain current workplane

* Exposed Area.Shapes attribute to return the current holding children
shape.