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
Sketcher Ellipse bug fix: Jump back on mouseup when shrinked
Sketcher Ellipse and ArcOfEllipse: Major bug fixes
- Change mode so that focus1 is not a point, but two doubles so that visual model and solver model match in number of points.
- Solver fix to deal with reduced constraint partials accuracy (threshold for matrix rank calculation tweak)
- Changes suggested by logari81
Sketcher Ellipse bug fixes:
- Fix of internal geometry creation
- Fix swapped negative and positive ends of major radius line on creation
- Removing the fudge factor (no longer needed)
- Fix to restore internal alignment - autodecide on constraint creation which point of the line is closer to the position
Sketcher Ellipse and ArcOfEllipse: Extra features and bug fixes
- Minor changes to alignment constraint selection to avoid to create objects just for simple calculations
- Equality constraint of Ellipse, ArcOfEllipse and combinations of those elements
- Bugfix for internalalignment creation of major and minor of ellipse and extension to arcofellipse
- Trim support for ellipses
- Solver threshold for rank calculation set at 1e-13.
- Trim support for arc of ellipse
- Ellipses and ArcOfEllipses as external geometry
- Validate Sketch now supports arcs of ellipse
- Tangents of Ellipse or ArcOfEllipse to any of ArcOfcircle, circle and Ellipse (using construction elements)
- Perpendicularity constraint
- Bug fix: Show/hide internal geometry not working for external geometry
- Visualization of Internal alignment constraints when selecting in the constraint widget
- Equality for ellipses and arc of ellipses
Sketcher Ellipse bug fix:
- Fix placement of equality symbols at ellipses and arcs of ellipses
Sketcher Ellipse and ArcOfEllipse: Extra features and bug fixes
- Visual representation of equality constraint: Separate circle and arcofcircle implementation from ellipse and arcofellipse implementation
- Fix for Alignment constraint: Now it allows to assign a second element, one by one
- Box selection for ArcOfEllipses
  Fixes the bug that vertex far away got selected due to lack of sync between the selection and draw loops.
Sketcher: Ellipse enhancement: Negative focus on creation to improve dragging

Focus2 is thrown in automatically for ellipse arcs

(cherry picked from commit 1ab814e6b07fd946dbb2910cc1bfb8588c61afb8)
Sketcher: Ellipse enhancement: All internal geometry is shown on creation

Design decision:
Elements that have internal geometry are created with the internal geometry on creation.

It has been under discussion for a long time. Ulrich was in favour (asked for it several times).
DeepSOIC implemented the focus because he hated the poor dragging of the ellipse when empty.
Squashable cleanup I
squashable clean up

remove // TODO: Ellipse
support for ellipses and elliptical arcs in flattened SVG output