Maya: 1. AssetManager: First rough implementation of threaded prop alembic export. Refactoring and checks tbd. 2. Fixed asset_manager.get_hashstring_from_list() method to hopefully have auto update of models correct now under all circumstances. 3. Added selection of proxy, rendergeo and locator for prop context menu. 4. Removed initialize_standalone() method from global_functions.py since it didnt work.