That requires some re-implementation of QPainterPath features, like
`constains`.
The usage of `QPainterPath::operator-` has been replaced by
`Face::operator^` because it's easier to implement and gives the same
result if one path contains the other.

# Conflicts:
#	src/path/face.cpp
#	src/path/face.h
#	src/properties/CMakeLists.txt
#	src/properties/splineproperty.cpp
#	src/propertytypeenum.h
#	src/renderers/offscreenrenderer.cpp
#	src/variant.h

improves error messages if it's used in the wrong way.

don't expect options parameter, options are the same for all tests.

Face operator^ is principally impossible to implement because
edges don't have an identity (there might be multiple indistinguishable
edges between two points).

Therefore, PathVector::faces is simpler now. It's still not correct,
but the (now removed) complicated implementation (employing operator^)
wouldn't make it better.
Simple and wrong is better than complicated and wrong.

Fixing it properly probably requires a complete change of paradigms
(i.e., identifiable edges)

That's going to be fixed on another branch `edge-identity`.