Part material variations

I’ve implemented a system that now automatically generates variations of all the generic block / plate parts with a selection of different materials.  These materials are just test examples for now, and are not final.

To make this work, I had to change the part ID convention, so I’m afraid any old save games will no longer work!  The good news though is that this part ID change should more easily allow modding in the future.

Now that there are more parts available (and eventually I plan to add many more), it’s starting to make the inventory UI a bit cumbersome to use.  At some point I think I’ll change it so that you first pick the block size, and then pick the material from a sub-menu.  First though I want to port over to Unity’s new UI system before making any more UI changes.