Ids not consistent when reloading

[BenjaminDeleze]

Describe the Bug
The ids of the same world change whether:

• We open Webots with this world for the first time.
• The subsequent reloads.

Steps to Reproduce

1. Open Webots
2. Open accelerometer.wbt
3. Close Webots
4. Open Webots
5. Export a scene locally
6. Reload the World
7. Export a scene locally
8. Open both scene files
9. The ids are differents

For example, here is what I get for the top level node:

First Time	Reload
2	1
4	3
5	4
6	5
16	15
156	155
157	156
168	167

Note:

• It does also apply to internal node, not just for the scene export.
• The difference of numeration is always the same for the same world
• The difference of numeration is not always the same between worlds (could be more than 1)