Implement: Basic Pick and Drop System

[AiraldiSebastian]

Basic Pick and Drop System

A basic pick and drop system for our FPS should offer at least the following functionality:

1. • Pick up items
     • When picking up items, these should disappear from the environment and
       be available at the players inventory
2. • Drop items
     • When dropping items, these should disappear from the players inventory and appear
       at the players location (feet) in the environment.

The inventory system should take care of calculating if the item can be picked-dropped or not.

This list is still subject to possible changes

[AiraldiSebastian]

This will be implemented in an abstract class called "Pickables", a first very basic implementation has been added in commit 4d5b370.