Alignment indicators

When aligning a selected part to another (frozen) part, prior to attachment, the game now shows where the available attachment points are.  These indicators reflect the type of attachment, either “surface” (e.g. as in block to block) or “interior” (e.g. as in block to axle).  The idea is that this should make it easier to accurately position parts where you want them.

Other stuff

In order to implement the alignment indicators I had to refactor a bunch of code, one side effect of which was that I was able to slightly reduce the per-rigidbody network data by calculating more stuff on the client side.  I also fixed a bug in explore mode, that was causing parts to not spawn.