AngleViaPoint: internal/external tangency locking

*Added automatic tangency type lockdown for all new constraints (only
for point-wise tangency).
Tangency type is stored in the constraint datum field, as an angle value
shifted by Pi/2 (to be able to treat 0.0 as undefined type).
Added ability to switch the tangency by setting datum value from python
(can be abused by passing arbitrary angle).
Further simplified the tangency related code in Sketch.cpp.