Use Part.FindModelTransform to find the launch transform.

It turns out it works nicely. I don't know why I had trouble with it with
the auger (the reason I avoided the function in the first place).

This fixes the launch transform not being found when MODE{} is used.
Add support for building sub-assemblies.

If the user can't do anything with the subassembly, that's his own tough
luck. However, this is a nice proof-of-concept for adding support for
categorized craft.
Center the build craft on the pad/dock.

While I can understand the desire to be able to shift the vessel off the
side of the pad, sub-assemblies pretty much require this because they get
saved with a ridiculous offset due to the way the editor works.