[Bug] Duplicating a level offsets the indexes of every level after it in the world

[Tourmi]

Steps to reproduce:

1. Create a room (Level_0)
2. Create another room (Level_1)
3. Right click on Level_0, and press "DUPLICATE"
4. Level_1 is now renamed to Level_2, the newly duplicated room now being called "Level_1"

What's expected:

• The duplicated room should have taken the Level_2 ID instead.

This is an issue since it makes merging changes very awkward, especially when saving levels to separate files.

[Tourmi]

Looked a bit more into it, and this is more of an issue with a lack of permanent ID available in the Default Level Identifiers. Having the IID or the UID in the filename would make this a non-issue. Opened a feature request instead #829