This adds a `Convert Curves to Legacy Hair` operator. It converts all selected curves objects into new particle hair systems on their respective surface objects. Existing particle systems with the correct name are updated, otherwise new a new particle system is added.
The purpose of the operator is to make the new curves sculpting tools useful even before all functionality is ported over.
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Currently, the generated hair system does not map exactly to the curves object for reasons that I don't fully understand. It's very close though. It might be a drawing issue or the hair strands are resampled somewhere else.
Questions:
* Should this always create a new particle system, or should it update an existing system? If it should update an existing system, should the mapping be by name?
* How should the operator be called?
* Where in the ui should the operator be located? (currently it is only available via search)