Add "remove overlap" function

[justvanrossum]

• It should take the selection into account:
  • Only work on selected contours
  • Could we make it work on a partially selected contour? This may be hard to do.
• We do not have JS implementation (long term I hope Kurbo + JS bindings will be the solution) so we are for now stuck with doing it in Python:
  • Send the selected outlines to the server, perform remove overlap, send result back (as we do with clipboard conversion and glyph name / code point lookups)
• Use skia-pathops rather than booleanOperations. The former supports quadratics. But perhaps we should be able to use both, as the tradeoffs between skia-pathops and booleanOperations aren't super clear-cut.