Started retro tetris game

Started developing a retro version of Tetris, following
the planning on the board of the classroom
(see planning.jpg).

Moved cursor work into cursor.

manager.py

@@ -0,0 +1,24 @@
+from piece import Piece, PIECE_DEFINITIONS
+from random import choice
+from retro.errors import AgentNotFoundByName
+
+class Manager:
+    """The Manager takes care of stuff that isn't anyone else's responsibility:
+    - Create a Piece whenever none exists.
+    - Clear full rows of Blocks (and move other Blocks down).
+    - End the game when the Blocks pile up all the way.
+    """
+    display = False
+
+    def play_turn(self, game):
+        try:
+            game.get_agent_by_name("piece")
+        except AgentNotFoundByName:
+            self.create_piece(game)
+
+    def create_piece(self, game):
+        width, height = game.board_size
+        piece = Piece((width//2, 2), game, choice(PIECE_DEFINITIONS))
+        game.add_agent(piece)
+
+