Set ship label on SGModel ships

So you can set the ship registration label. I've tried to do this in the least intrusive way, so its still driven from `ShipFlavour`. Fixing that is a job for another time.

![](http://i.imgur.com/C8qgxZV.png)

![](http://i.imgur.com/2LYwMxJ.jpg)

(The black box is an uninitialised decal, fixed in #1976. The hyphen being down on the baseline in the second image is a problem with `DistanceFieldFont` ignoring glyph offsets. I'm working on that now and will have a PR for it soon).