Synthesis of Technic Universal Joints (#456)

* Split synth info initialzation by type.

We are going to remove the type enumeration and use a class hierarchy
instead. This preparation will then be helpful.

* Make Add...Parts() overrides of a virtual AddPart() function.

Since we have a class hierarchy for the different synthesized pieces, we
can now turn a case distinction into a virtual function call.

* Move initialization based on type to derived class constructors.

Move initialization of end transformations of flexible parts into
class lcSynthInfoCurved.

* Make GetDefaultControlPoints() virtual with overrides.

* Remove obsolete enum lcSynthType.

We have replaced its purpose by derived classes by now.

* Initialize shock absorbers' spring part ID early.

This removes the awkward early return that is needed in the if-else
cascade.

* Split lcSynthInfo into derived classes for curved and straight pieces.

* Only curved parts have varying sections, start, middle, and end properties.

Move the properties from the base class to the derived class that needs
them.

* Use derived classes to mark synthesized objects of different kinds.

We will extend the derived classes in the upcoming commits.

* PieceInfo is only needed to synthesize some hoses and shock absorbers.

* Initialize edge part IDs of flexible hoses early.

This removes another case distinction in AddParts().

* Verify the number of control points loaded from a model file.

* Synthesize Technic universal joints.

The direction of one end can be changed so that it points to the control
point.

* Technic universal joints need only the position of the control point.

* Synthesize legacy universal joints.

common/lc_model.cpp

@@ -684,6 +684,7 @@ void lcModel::LoadLDraw(QIODevice& Device, Project* Project)
 				Piece->SetPieceInfo(Info, PartId, false);
 				Piece->Initialize(Transform, CurrentStep);
 				Piece->SetColorCode(ColorCode);
+				Piece->VerifyControlPoints(ControlPoints);
 				Piece->SetControlPoints(ControlPoints);
 				AddPiece(Piece);
 				Piece = nullptr;