Racks: Allow to flip and adjust unit-numbering

[neufeind]

There is a boolean to indicate descending unit-numbering (implemented with #191). But once you change that flag for an existing rack all devices change their positions as well and the rack gets flipped upside-down (technically correct). Would it make sense to allow flipping rack-numbering once you toggle that boolean? And maybe to be able to mass-adjust numbering by +/- a few units, just in you figure out you missed to count as well one unit on the top or bottom as well?

[DanSheps]

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