Changes to the name of the `navmesh` and `navpoly` properties on
`NavigationRegion` caused navigation data to be lost on load.
This PR creates uses `_set`/`_get` to handle compatibility with the
older names on load, preserving the data.
Also fixes a typo on `get_vertices_per_polygon` in `NavigationMesh`,
and renames the property to remove the `polygon_` prefix which doesn't
match the setter/getter.
Co-authored-by: Rémi Verschelde <rverschelde@gmail.com>
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