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/AppPart.cpp

@@ -66,6 +66,7 @@
 #include "EllipsePy.h"
 #include "ArcPy.h"
 #include "ArcOfCirclePy.h"
+#include "ArcOfEllipsePy.h"
 #include "BezierCurvePy.h"
 #include "BSplineCurvePy.h"
 #include "HyperbolaPy.h"
@@ -172,6 +173,7 @@ void PartExport initPart()
     Base::Interpreter().addType(&Part::ParabolaPy           ::Type,partModule,"Parabola");
     Base::Interpreter().addType(&Part::ArcPy                ::Type,partModule,"Arc");
     Base::Interpreter().addType(&Part::ArcOfCirclePy        ::Type,partModule,"ArcOfCircle");
+    Base::Interpreter().addType(&Part::ArcOfEllipsePy       ::Type,partModule,"ArcOfEllipse");
     Base::Interpreter().addType(&Part::BezierCurvePy        ::Type,partModule,"BezierCurve");
     Base::Interpreter().addType(&Part::BSplineCurvePy       ::Type,partModule,"BSplineCurve");
     Base::Interpreter().addType(&Part::OffsetCurvePy        ::Type,partModule,"OffsetCurve");
@@ -261,6 +263,7 @@ void PartExport initPart()
     Part::GeomBSplineCurve        ::init();
     Part::GeomCircle              ::init();
     Part::GeomArcOfCircle         ::init();
+    Part::GeomArcOfEllipse        ::init();
     Part::GeomEllipse             ::init();
     Part::GeomHyperbola           ::init();
     Part::GeomParabola            ::init();