Point on polygon

[hilaire]

Define a point on a polygon.

Polygon item are already capable to define a point according to its curvilinear coordinate in [0 ; 1]

[hilaire (hilaire-fernandes)]

In Polygon item:

- #abscissaOf: should be implemented
- isCurveItem should return true

In Polygon costume:
- isCurveCostume should return true

[hilaire (hilaire-fernandes)]

Implementing like this seems to work:

abscissaOf: aPoint
 ^ (self closestPointInfoTo: aPoint) second

It may need more testing, so I did not commit any code now.

[hilaire (hilaire-fernandes)]

The intersection tool broke when selecting a line and a polygon.
For now we should make polygon unselectable for intersection.