Implement qcurve handling

[madig]

Continuation of the work in https://github.com/trufont/trufont/pull/599.

One test case: open a font with qcurves (e.g. Ubuntu-B.ufo) and try to write text in the font view.

Note to self: look at fontTools' decomposeQuadraticSegment function to generate implied on-curves, compute bounds for one sub-segemnt and unionize all bounds.