#76 vertex extrude: one more action for this operation

[Dominique B.]

At the moment the user can only by using the "blue
arrow" determine how far away the new vertex (top) will
be. The new vertices of the base are fix - i think half
of the their edge? Would be better to be able to change
their positions interactively.

In 3ds max vertex extrude works like this for the user:

- activate mode
- go over the vertex an click and drag (if it's one of
several selected vertices, then do operation for every
selected vertex, else only for the vertex under the
mouse pointer)
- dragging in the vertical slides the top vertex away
from the base (or back to base)
- dragging in the horizontal (talking about the view
coords) determines the 'scale' of the base.