[Piece] Piece Generators

[mbrown1413]

The idea of a Piece Generator is to allow for many pieces to be generated easily. Some ideas:

• Enumerate all pieces of a given size
• Enumerate all ways a piece could be split
• Split a single "main" piece based on a voxel attribute (such as a color). This could be useful in starting with a goal shape, then carving it out by assigning attributes. (going a different route to achieve this: piece assemblies)
• Piece which uses the final placements of a problem's solution as a new piece

The pieces list will need to support nested items (#36). Both normal pieces and generators would be top-level, but generated pieces would appear nested under a generator. Selecting the generator would show a form to edit the generator instead the typical editor. Selecting a piece in the generator would show a (read-only?) item as usual.

[mbrown1413]

This issue used to cover both piece and problem generators, but now it only covers piece generators. If we have a good use-case for problem generators I'm open to adding another issue for it.

Also to note, indirect progress is being made on this. The concept of a piece group is coming together to support piece assemblies, and I plan on making generators another type of piece group.