Started to work on Collisions

Right now, the player can move around. Elements marked with red =
collides with player

Added Player moving
Added Physics class
Added World.GetPhysics
Added new layer WORLD_BACKGROUND (only child is the background sprite -
is neccessary because otherwise entities would collide with it)
Added Physics.GetObjectsIntersecting for FloatRect and for SceneNode

Added virtual SceneNode.GetBoundRect
Added SpriteNode.GetBoundingRect, renamed SceneNode.SetRect to
SceneNode.SetBoundingRect (accepts FloatRect & IntRect)
Added SceneNode.CollidesWith

Added SpriteNode.GetTexture
Updated SceneNode's IsX functions