* spot/twa/twa.cc, spot/twa/twa.hh: Add a way to
remove named properties.
* spot/twa/twagraph.cc: Clear highlight-edges on operations
that reorder the edge vector.
* spot/twaalgos/randomize.cc, spot/twaalgos/randomize.hh:
Preserve highlighted state, but not highlighted edges.
* spot/twaalgos/hoa.cc: Adjust output of highlight-edge
when the edges are not stored in order.
* tests/core/readsave.test, tests/core/tgbagraph.test,
tests/core/twagraph.cc: More test cases.