Sketcher New Feature: Ellipse support

- Ellipse introduction button via (center,majaxis extreme, a point in edge), ellipse is always CCW so that Z axis goes in the positive direction of the sketch
- Backwards compatibility with files of previous versions of ellipse not defining a phi angle
- Art by Jim (all the icons you see and the XPMs shown on creation of an ellipse)
- Element Widget support for ellipses
- Box selection for ellipses
- Point on Ellipse constraint based on the gardener's method based on Ulrich's function proposal (radcan simplified, i.e. with simplify_radical sage function)
- Tangent: Ellipse to Line based on DeepSOIC's geometric formulation (radcan simplified)

Sketcher New Feature: Internal Alignment Constraint
- The element to which internal alignment is applied has to be selected last.
- All other elements are added in the order of priority, taking into account existing elements
- Art by Jim (beautiful icons).

Sketcher New Feature: Tool to show/hide/restore the internal geometry of an element
- New functionality for show/hide internal geometry:
  toggles between hiding all unused internal geometry elements and showing all internal geometry.
  The restore function is implicit to the showing all internal geometry

Sketcher New Feature: Arc of Ellipse support
- Part::Geometry + Python implementation
- ArcOfEllipse creation method
- Art by Jim (all the icons you see and the XPMs shown on creation of arc of ellipse elements)
- Sketcher Element widget for ArcOfEllipse.

Bug fix: Select elements associated to constraints works now for foci internal alignment constraints

src/Mod/Part/App/EllipsePyImp.cpp

@@ -151,6 +151,38 @@ void EllipsePy::setMinorRadius(Py::Float arg)
     ellipse->SetMinorRadius((double)arg);
 }
 
+Py::Float EllipsePy::getAngleXU(void) const
+{
+    Handle_Geom_Ellipse ellipse = Handle_Geom_Ellipse::DownCast(getGeomEllipsePtr()->handle());
+    
+    gp_Pnt center = ellipse->Axis().Location();
+    gp_Dir normal = ellipse->Axis().Direction();
+    gp_Dir xdir = ellipse->XAxis().Direction();
+        
+    gp_Ax2 xdirref(center, normal); // this is a reference system, might be CCW or CW depending on the creation method
+    
+    return Py::Float(-xdir.AngleWithRef(xdirref.XDirection(),normal));
+
+}
+
+void EllipsePy::setAngleXU(Py::Float arg)
+{
+    Handle_Geom_Ellipse ellipse = Handle_Geom_Ellipse::DownCast(getGeomEllipsePtr()->handle());
+
+
+    gp_Pnt center = ellipse->Axis().Location();
+    gp_Dir normal = ellipse->Axis().Direction();
+    
+    gp_Ax1 normaxis(center, normal);
+    
+    gp_Ax2 xdirref(center, normal);
+    
+    xdirref.Rotate(normaxis,arg);
+    
+    ellipse->SetPosition(xdirref);
+
+}
+
 Py::Float EllipsePy::getEccentricity(void) const
 {
     Handle_Geom_Ellipse ellipse = Handle_Geom_Ellipse::DownCast(getGeomEllipsePtr()->handle());