Sketcher: Viewprovider B-Spline pole representation factor

==========================================================

Previously for Weights, ViewProviderSketch used getScaleFactor. This caused that upon zoom change
the Weights would not increase progresively, rather the would grow on the next redraw. Additionally,
upon substantial zoom out, the poles would grow several times bigger than the B-Spline.

This commit uses a new geometry extension intended only for ViewProviderSketch, to store a geometry
specific representation scale factor. This is calculated as a function of the B-Spline length. The
extension does not serialise to disk. It is just intended for runtime.

Dragging from the edge when the radius is constrained gives a wrong cosmetic result, because the representation circle and the
real value of the weight are different (by a scale factor). This commit prevents dragging on the edge in the most representative
cases where the radius is constrained.

src/Mod/Sketcher/Gui/ViewProviderSketchGeometryExtension.cpp

@@ -0,0 +1,79 @@
+/***************************************************************************
+ *   Copyright (c) 2019 Abdullah Tahiri <abdullah.tahiri.yo@gmail.com>     *
+ *                                                                         *
+ *   This file is part of the FreeCAD CAx development system.              *
+ *                                                                         *
+ *   This library is free software; you can redistribute it and/or         *
+ *   modify it under the terms of the GNU Library General Public           *
+ *   License as published by the Free Software Foundation; either          *
+ *   version 2 of the License, or (at your option) any later version.      *
+ *                                                                         *
+ *   This library  is distributed in the hope that it will be useful,      *
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of        *
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the         *
+ *   GNU Library General Public License for more details.                  *
+ *                                                                         *
+ *   You should have received a copy of the GNU Library General Public     *
+ *   License along with this library; see the file COPYING.LIB. If not,    *
+ *   write to the Free Software Foundation, Inc., 59 Temple Place,         *
+ *   Suite 330, Boston, MA  02111-1307, USA                                *
+ *                                                                         *
+ ***************************************************************************/
+
+#include "PreCompiled.h"
+
+#include <Base/Writer.h>
+#include <Base/Reader.h>
+#include <Base/Exception.h>
+
+#include "ViewProviderSketchGeometryExtension.h"
+
+using namespace SketcherGui;
+
+//---------- Geometry Extension
+TYPESYSTEM_SOURCE(SketcherGui::ViewProviderSketchGeometryExtension,Part::GeometryExtension)
+
+
+ViewProviderSketchGeometryExtension::ViewProviderSketchGeometryExtension():RepresentationFactor(1.0)
+{
+
+}
+
+
+// Persistence implementer
+unsigned int ViewProviderSketchGeometryExtension::getMemSize (void) const
+{
+    return sizeof(double);
+}
+
+void ViewProviderSketchGeometryExtension::Save(Base::Writer &writer) const
+{
+    (void) writer;
+    // So far only intended for runtime
+}
+
+void ViewProviderSketchGeometryExtension::Restore(Base::XMLReader &reader)
+{
+    (void) reader;
+    // So far only intended for runtime
+}
+
+std::unique_ptr<Part::GeometryExtension> ViewProviderSketchGeometryExtension::copy(void) const
+{
+    auto cpy = std::make_unique<ViewProviderSketchGeometryExtension>();
+
+    cpy->RepresentationFactor = this->RepresentationFactor;
+
+    cpy->setName(this->getName()); // Base Class
+
+#if defined (__GNUC__) && (__GNUC__ <=4)
+    return std::move(cpy);
+#else
+    return cpy;
+#endif
+}
+
+PyObject * ViewProviderSketchGeometryExtension::getPyObject(void)
+{
+    THROWM(Base::NotImplementedError, "ViewProviderSketchGeometryExtension does not have a Python counterpart");
+}