RenderItem: move out of renderer.entity package

[RainWarrior]

it's not an entity renderer (that's RenderEntityItem), but a general item-rendering-related class.

[LexManos]

RenderItem and ItemRenderer are two cloosly linked classes, Yes this one needs to be moved out of entity however they should both be renamed to something better.
Please suggest names here guys.

For now those, I've moved it out of entity, into the root renderer package.
We need Better names tho.

[kashike]

ItemRenderer seems to handle items in a player's hands/arms

[mezz]

Name change suggestions:

net.minecraft.client.renderer.ItemRenderer --> FirstPersonRenderer
Half the methods in ItemRenderer end with InFirstPerson which could be simplified if the class was named with it.

net.minecraft.client.renderer.RenderItem --> ItemRenderer
RenderItem is pretty consistently stored in fields named itemRenderer.