Sketcher: Changes to split edge functionality

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This commit is directed to external functionality of the split() function.

1. getAppliedConstraints renamed to getConstraintIndices

This is just for clarity being a general function

2. SwapInvolvedGeometry functionality moved to Constraint class

Why?
i. Because it is a specific operation on a constraint, it must not be
a public function, as it does not define interface of the Sketch.
ii. It could be a lambda or a private utility function, but them it would not be reusable.
iii. It could be part of a helper class, but then, it is would be less reusable.

3. renaming of the flag passed to transferConstraints function

src/Mod/Sketcher/App/SketchObjectPyImp.cpp

@@ -570,7 +570,7 @@ PyObject* SketchObjectPy::delConstraintOnPoint(PyObject *args)
     if (!PyArg_ParseTuple(args, "i|i", &Index, &pos))
         return 0;
 
-    if (pos>=0 && pos<=3) { // Sketcher::none Sketcher::mid
+    if (pos>=Sketcher::none && pos<=Sketcher::mid) { // This is the whole range of valid positions
         if (this->getSketchObjectPtr()->delConstraintOnPoint(Index,(Sketcher::PointPos)pos)) {
             std::stringstream str;
             str << "Not able to delete a constraint on point with the given index: " << Index