Untangle: add cursor control interface.

The cursor keys navigate amongst the points. CURSOR_SELECT toggles dragging;
CURSOR_SELECT2 and the Tab key cycle through the points.

The cursor navigation scheme jumps to the nearest point within the quadrant
of the cursor direction; this seems to yield fairly intuitive gameplay.
Unfortunately, the "quadrant-nearest-neighbors" digraph produced by this
scheme is not necessarily fully reciprocal; that is, pressing opposite
cursor keys in sequence does not always return to the original point. There
doesn't seem to be any immediately obvious way around this.

As for connectivity (i.e. whether all points are reachable from any given
point), I could not find a counterexample, but I don't yet have a formal
proof that this is the case in general. Hence, I've added the ability to
cycle through all the points with Tab. (This will probably also be useful
in conjunction with the "Numbers" point drawing preference.)

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