Change the way change detection is enabled & disabled again

[kaosat-dev]

Up until now, change detection was

• disabled before an export (so that all the changes we make are not registered)
• enabled again with a one second timer after the call to export

Turns out, based on the newly added better change detection tests:

• that they do not get enabled again correctly , at least in cli mode (timer does not run ?)
• but also very importantly, that the logic behind enabling them again is WRONG : it should only happen after the export is REALLY done: so we should switch this to use the post export hooks (glTF2_post_export_callback)!

This could potentially explain the observed semi random weirdness around change detection ! (both it not working for some folks, and running too much for others !)