Description

This PR keeps all legacy names in the PROPS dictionary. No existing keys are renamed and any newly deprecated names have a new key inserted.

As a single, concrete example:

'LocalCentroid': 'centroid_local', 'local_centroid': 'centroid_local',
Here LocalCentroid was the property name in old scikit-image <= 0.8 and local_centroid is the current property name we want to modify. In both cases, the value should be the new, documented property name: centroid_local. Users should be able to use any of the 3 names to get this property.

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.