Shape builder fixes

A large batch of fixes for the Shape Builder. Fixes most open issues, as well as many unreported bugs. The MR largely consists of making better use of livarot's functionality. The remaining issues (e.g. spurious shapes, bits of line segments and tiny pieces) are probably unfixable without replacing livarot.

As an example, the following shapes now behave properly in the Shape Builder:

Using a hack to show distinct shape builder regions in random colours, this results in