Adds option to disable new invlets

Empty invlet assignment with full inventory can be a bit annoying at times, such as when you need to drop your melee weapon, unload a battery tool (batteries will take old invlet) or tend to run out of a particular type of consumable (say, you constantly empty your clean water bottle).

This adds an option to disable empty invlet assignment, so invlets will only be assigned from favorite cache (and manually by the player).