BREAKING ContourModel::AddVertex and refactored ContourElement and fixed memory management

+ Fixed T28148
+ Added tests for ContourElement
+ Extended some test for ContourModel
+ Improved constness of ContourModel interface
+ Updated some function documentations of ContourModel

Reason for breaking:

• Aligned signature of AddVertex and AddVertexAtFront. First offered passing vertex by pointer, later by reference.
• Picked "by refernce" to make it clearer, that not the passed vertex is added but a clone of it.

How to migrate:

• Use dereference operator (*) if you want to pass a vertex pointer to ContourModel::AddVertex();