Restyling graph backedges + bring back 't' and 'f' hints

[cyanpencil]

1. Bring back t and f hints in graph. Governed by variable graph.hints. Defaults to True.
   Added command # to toggle hints while in graph view.

2. Use a different style for backedges.
   I tried using braille characters, but the result was not good (braille points are a 2x4 table, unfortunately all even numbers, lines were decentered and the visual effect was worse than normal hyphens (-)).
   So I ended up using dashed lines for backedges. I think it is the most clear way to differentiate them from both normal edges and conditional edges. Take a look in this screens:

On a different terminal with different font, a conditional backedge:

[cyanpencil]

For reference, this is how it would have come out with Braille chars:

I don't know, there are small problems in the corners and near the arrows, becuase the points are not centered in the terminal cells, for this reason I chose to go with dashes.

Tell me if you think braille looks better to you

[Maijin]

Not really fan of t and f by default, i find it looks way cooler with the boxes connected to the lines.
Probably need a vote or something to decide what default should looks like @XVilka ?