Bug fixes, piece selection changes

No new levels/mechanics for this one.

• Fixed the issue where undoing a piece that had been pushed into a hole caused it to not change sprites back.
• Selection has been reworked. You can no longer select pieces that cannot be moved (triangles, pieces already in holes). When a selected piece enters one of these states, the next piece is chosen automatically.  Should improve the gameplay flow :)
• Level numbers added on the bottom right of each level scene