Ruleset and randomizer refactoring (Read comments)

You can now specify an arbitrary number of pieces for a ruleset.
The randomizers will adjust accordingly.
Expect a pento ruleset in the modpack soon!
Also, gamemode skin selection has been refactored.

tetris/randomizers/randomizer.lua

@@ -3,6 +3,7 @@ local Object = require 'libs.classic'
 local Randomizer = Object:extend()
 
 function Randomizer:new()
+	self.possible_pieces = 7
 	self:initialize()
 end