EntityCoordinates are BlockCoordinates.

Everything starts with Coordinates. They have x, y and z.
Now FacingCoordinates adds pitch&yaw and BlockCoordinates add a world.
EntityCoordinates also adds a world, but is based on FacingCoordinates.
Conclusion: EntityCoordinates = FacingCoordinates + BlockCoordinates

This also cleans up the mutable interfaces a bit.

Old inheritance graph:
![pluginbase-location-inheritance](https://f.cloud.github.com/assets/754850/447084/afa315d0-b1ea-11e2-88fb-ef96195344a4.png)

New inheritance graph:
![pluginbase-new](https://f.cloud.github.com/assets/754850/447091/f3406fc2-b1ea-11e2-8e03-60618bf8f734.png)

I noticed this while fixing the MV3 location code that was broken in 304dfe64676033b0d8faa183754d5dc7c0f4b591.