Create duplicated slots above the selected range when duplicating multiple slots

Currently, when duplicating multiple slots, each new slot is always created directly above its original slot. For example, if the following slots are selected and duplicated:

1. slot A
2. slot B
3. slot C

The resulting order is:

1. slot A2
2. slot A
3. slot B2
4. slot B
5. slot C2
6. slot C

However, when multiple slots are selected together, it may be more convenient to treat them as a single set and place all duplicated slots above the entire selected range, resulting in the following order:

1. slot A2
2. slot B2
3. slot C2
4. slot A
5. slot B
6. slot C

This could make subsequent organization and editing easier, so there may be room to improve the current duplication behavior.

Related forum thread:
https://esotericsoftware.com/forum/d/30450-%E6%80%8E%E4%B9%88%E6%8A%8A%E9%AA%A8%E9%AA%BC%E6%94%BE%E5%9C%A8%E6%96%87%E4%BB%B6%E5%A4%B9%E5%86%85%E4%B8%80%E8%B5%B7%E5%90%8C%E6%97%B6%E5%A4%8D%E5%88%B6%E9%AA%A8%E9%AA%BC%E5%92%8C%E6%8F%92%E6%A7%BD

Start by reproducing multi-slot duplication in the Spine editor with a contiguous selection and compare the current order with the requested order. Done means duplicated slots appear together above the selected range, in the same order as the originals, without changing single-slot behavior.

Written by the indexing model from the issue text.