Add an option to change unit sorting order.

[dsa-t]

Currently, units are sorted in this way:
A, AA, AB, AC, B, C

It would be nice to add an option to control the unit sorting order:

1. Do not sort (units should appear in the same order as in the CSV file)
2. Sort like in KiCad (A, B, C, AA, AB, AC)
3. Preserve current sorting order (A, AA, AB, AC, B, C)

Currently, the workaround to achieve KiCad-like sorting order is to rename double-letter names to something like:
A, B, C, ZA, ZB, ZC

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Forked from issue #66