make Draft.BezCurve nodes tangent/symmetric
Draft: Bugfix in _BezCurve.symmetricpoles
Draft edit mode: only allow one mode at a time (add,del,symm,tang)
Draft bezier tangentpoles

instead of moving both poles to the same side, we make them symmetric
changed icon Draft_BezTanNode
Draft: prepare functions to edit tangent or symmetric bezier curves