Drawing system + walls as entities

- Renamed "position" to "positionable"
- Added "drawable" class. Includes player and wall.
- Added wall class.
- Reworked drawing to use a DrawingSystem system instead.
- Centralized Curses error catching for last (79, 24) char in console
- Screens just draw dots for now.
- Removed some old cruft related to updating and elapsed_seconds