add display_context pointer to resources

This should be removed soon, it is just a short term corrective
measure for an editor segfault introduced in prior commits. The
plan is to correct the segfaults, then refactor this away.
fixup segfaulting editor, favoring display_context over game_board

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.
Merge branch 'fixup_segfaulting_editor'