Inventory preparations

With this PR I am beginning the preparations for the fully-fledged inventory system. Every body part and equipment item will have a set of provided capabilities (e.g. `ManipulatorCapability` meant to be used for arms and tentacles body parts, and any robotic manipulator equipment). Equipment items will also have a set of required capabilities that should be satisfied for the equipment item to be used.

I am also introducing the concept of _equipping_ an item. My next step is to create the corresponding action for "equip an item". It will be triggered when player uses the new (to be implemented) inventory screen. It will allow the player to manage capabilities of his equipment (he can even provide new capabilities by equipping some items).