Clean up the names of entity methods

[rexim]

Go through all of the entities (game, level, player, etc) and make sure they follow the convention:

• One translation unit per entity
• Constructors are create_<entity-name> or create_<entity-name>_from_<resource-name> (for example create_level_from_file)
• Destructors are destroy_<entity-name>
• Regular methods are <entity-name>_<method-suffix> (for example player_render)
• Document these conventions somewhere