* Reviewed subtypes, making them more specific and adding new ones.
* Subtypes now have an associated type of units (length, area, volume,
mass, rotation, time, velocity, acceleration). These are not used
yet anywhere.
* Centralized code that decides the name of array items based on
subtype (XYZ, RGB), was copied in 3 places.
* RNA_def_float etc functions still need to be update, will do this
later together with another change.
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