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/Sketcher/Gui/EditDatumDialog.cpp

@@ -72,8 +72,10 @@ void EditDatumDialog::exec(bool atCursor)
 {
     // Return if constraint doesn't have editable value
     if (Constr->Type == Sketcher::Distance ||
-        Constr->Type == Sketcher::DistanceX || Constr->Type == Sketcher::DistanceY ||
-        Constr->Type == Sketcher::Radius || Constr->Type == Sketcher::Angle) {
+        Constr->Type == Sketcher::DistanceX || 
+        Constr->Type == Sketcher::DistanceY ||
+        Constr->Type == Sketcher::Radius || 
+        Constr->Type == Sketcher::Angle) {
 
         if (sketch->hasConflicts()) {
             QMessageBox::critical(qApp->activeWindow(), QObject::tr("Distance constraint"),