Make consumables and items "attract" to the player

[cdsupina]

Consumables and items move straight down to the bottom of the arena and the player must move exactly over them to get them.

Let's make it slightly easier for the player to get items and consumables by adding an "attraction" component and system.

Entities with this component should be attracted to certain types of "target" entities. If a target entity moves into a defined radius, the entity should accelerate towards the target.