drawable unit inherits from unit, and implements the draw function,
used only by the display. when the display wants to use the function,
it casts a unit pointer to a drawable_unit, and draws it. this
improves encapsulation.
Currently, the editor does not generate game_board objects, and
leaves the pointer resources::gameboard null. This commit makes
map_location, reports, and tod_manager, three things used in
editor mode, use the display_context pointer instead of the
game_board pointer. We also move a function to display_context,
which finds a const unit * to a visible unit at a hex.
