ProfilePathFinding: use Albert's updater thread for responsivity

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
ProfilePathFindng: minor speedup tweaks.

Do not recompute min&max and source image every time we trigger a
computation, but only when we change slice.
I know, this should be a Johannes Fiji Tool. Later.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>