* minor tweaks & improvements to assets handling
* renamed a few auto export preferences for more coherence
* "assets" folder is now mandatory again and used by the various sub asset types
* blueprints/levels/materials paths in auto_export are not overwrite their relative values anymore,
but added their "full" variants & changed code using full paths to use those
* added back gltf exporter post export callback, in a cleaner fashion
* changing (wip) the way the various relative folders are auto generated in the folder selection UI
* lots of other minor tweaks & improvements
