Before this, after each move, the whole maze was redrawed to update
display. Now it draws only the changed tiles (current/old player
position). Changed tiles are stocked in a list, for future needs,
others changed tiles can be add here.
TODO keep maze.string updated
Manages graphic display with pygame
Move from `func.py`: set_`header()` & `maze_draw()` (renamed `draw()`)
Deletes `Maze.maze_print()` (integrated in `GraphicUI.draw()`)
Removes the background, using only tiles
TODO: Add a floor tile behind player, guard and items
Use pygame's display to setup a 15 cells square grid, with a player
moving on it. Do not clean the trail, border collision.
Next step: build an gui object for representing the maze.
Fred Z