Sketcher helper constraint

Helper constraint is a regular sketcher constraint that is needed as a part of a more complex constraint, but is exposed in user interface to assist dealing with redundancy. For example, for Snell's Law constraint, the two lines that represent rays of light need to be connected (coincident constraint), and the joint must lie on the interface (Point On Object consraint).

Helper constraints are added automatically when they are needed. The decision for if they are needed is currently made by evaluating the helper constraint error for current state of geometry (this may change in future versions). If the error is small enough, the constraint is considered to be unnecessary, and is not added. In some cases, this behavior can lead to errors (the constraint can be satisfied by accident, which can easily happen when Sketcher Grid Snapping is on).

If this happens (a helper constraint is missing, and the required conditions is not satisfied otherwise), the complex constraint will be broken. It will do something, but the actual behavior is undefined. Such a broken constraint can be repaired by adding the missing helper constraint manually.

Helper constraints are currently required for:
 * Constraint Tangent (in tangent-via-point mode; two point-on-object constraints are needed)
 * Constraint Perpendicular (in perpendicular-via-point mode; two point-on-object constraints are needed)
 * Constraint InternalAngle (in angle-via-point mode; two point-on-object constraints are needed)
 * Constraint SnellsLaw (coincident constraint and point-on-object constraint are needed)

Helper constraints were introduced in v0.15.4387