Previous behavior:
- They'd be sorted by default
- They'd derive display order if `DeriveDisplayOrder` was set
- This could be set recursively
- The initial display order value for subcommands was 0
New behavior:
- Sorted order is derived by default
- Sorting is turned on by `cmd.next_display_order(None)`
- This is not recursive, it must be set on each level
- The display order incrementing is mixed with arguments
- This does make it slightly more difficult to predict