Introduce POLYGONHOLE_MODE for creating holes in polygons

Introduce POLYGONHOLE_MODE for creating holes in polygons

Having selected polygon hole mode, the first click selects which
polygon to cut a hole in. A second click defines the start point of
the hole contour. The tool then behaves in a similar way to the
polygon drawing tool, with the hole ending when the start point is
re-clicked.

To avoid creating illegal polygons, the hole drawn is subtracted from
a representation of the original polygon with the poly_Boolean_free().
This consolidates any contours it intersects with and prevents the user
defining contours which intersect each other. (Although we don't
currently prevent the the user drawing self-intersecting contours).

The resulting POLYAREA is re-processed into PolygonType objects,
potentially more than one - if the hole drawn bisects the original
polygon. To keep undo operations simple, these are added as completely
new objects and the original polygon is deleted - along with its ID.