directed graphs fully implemented

- Press "D" to turn on directed graphs. arrowheads move appropriately.
turning DG mode off hides (not destroys) existing arrowheads, and
turning back on reshows them
- the flood, DFS, and BFS algorithms should (should) follow the
directions (only when DG mode is on)
- export to LaTeX now draws arrowheads if present (and if DG mode is on)
- color #7 (white) added