Comment 2 for bug 699435

Improved, rebased against current CVS.

Now the patched tool is expected to work like this:
1.point at the center,
2.the first point of arc
  (up to 270 degrees; sets the arc direction),
3.the second point of arc (the direction is the same as at the end
  of 2.; when the new arc overlaps the arc drawn in step 2., a full
  circle is created);
4.return to 3. until ESC (`F3' moves the starting point
  to end of the latest arc and returns to 3.)

During 3. and 4. Shift-click moves the center of the arc
to the pointer.

Switching from/to Line tool draws chained segments.