app, libgimp, pdb: add "distance-metric" property to GimpPDBContext.
This property is currently only used for gimp_edit_blend() to control how are computed distances. In the future, it could be used for more functions making use of "gegl:distance-transform" operation, or even for other algorithms, if relevant. This new property obviously comes with 2 new PDB calls: gimp_context_get_distance_metric() & gimp_context_set_distance_metric()
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