Fixed crashes in BridgeDetector due to unexpected implicit conversion to a const temporary.
Layer newly remembers bounding boxes of slices, the bounding boxes are used by G-code generator & newly the support generator. Slices are stored as ExPolygons, not ExPolygonCollection.
The XS was left only for the unit / integration tests, and it links libslic3r only. No wxWidgets are allowed to be used from Perl starting from now.