[Bf-funboard] move to target/edge constraint

[eugene]

It would be nice to be able to constrain the
translation of vertices to edges that they lie on.
A far-fetched example from Wings is the Slide function
for edges. Can this behaviour be adapted to vertices?
Another, better example is the Edge Constraint in 3ds
max. What happens is a vertex slides along an edge it
is part of. The edge is chosen automatically. This
works for multiple selected vertices as well.
A little thing like that can save lots of time and
ensure great precision.
If edge selection can't be made automatic in Blender,
this feature could have a cycle option for direction.
Direction could cycle between "perpendicular" edge
loops when possible, or between individual edges when a
loop terminates at a vertex.
Currently, things can helped by constantly aligning the
view to 3-vertex selections, but this is neither
intuitive nor fast.

I hope I've put it clearly. What do you think of this?