Added some new special pieces; pieces are now assigned integer IDs, which are stored in a list and sampled from when generating new ones
Added special pieces frame
Added an icon to the special piece frame
Set static colors for special piece displays
Piece conveyor belt is now reset upon game load
Added a method to the piece factory to return whether or not a special piece is currently active
Special pieces button is now appropriately disabled
Difficulty setting is now accessible only through settings panel instead of also storing it in the game board model
Pieces are now converted to an array from a set upon game start to make sampling easier
Minor changes to PieceSelectorButton class
Merge master into no-soundtrack