ProfilePathFinding: adapt slider value to image range

and do not operate on 3D volume if any.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
ProfilePathFinding: put plugin in the right menu entry location

Use your brain when you type on the keyboard!
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>