Missing features related to entities

[Unarelith]

Lighting:

• Item drop faces should have different colors (f926f4f)
• Better entity lighting

Lua:

• Entity Lua definition (proposal here, see Entity Lua API #100)
• Item drop entity spawn from Lua (Entity Lua API #100)
• Improvement of block callbacks to add player parameter (needed for cactus and sugar canes)

Physics and collisions:

• Collisions between entities and the world
• Gravity for item drops

Selection:

• Ability to select an entity
• Selection box around the entity
• BlockInfoWidget should be capable of displaying informations about entities

Player management improvement:

• Players should be part of the scene
• Move the item drop collision code from CollisionController

Visual effects:

• Entity shadow (not a real one, more like an image on the ground)