Maya: AssetManager: 1. Complete refactoring of menu creation. Moved entire menu creation into own module asset_manager_menu.py. Menu creation just got too big codewise, so it was a readability issue to encapsulate it. Used this occasion to double-check that all controls are initialized with the default values of the tool. This means it is safe to omit the menu on artists machines. 2. Refactored security thread count settings. Max threads is always all available cores now and if set_thread_count_to_half_of_max is set (default), active threads will be half of max threads. This rule is enforced at thread startup.