Add super basic player entity
Add plumbing for the most basic player: - A `Player` component to mark the entity which is the player. - A `Player` resource which holds the entity ID of the player. - A `draw_tmp_player_system` to draw the player as a circle (for now). - A `create_player` function to create the player entity. - Update the `moving_system` to put the camera at the player position.