shadows&highlights: a more reasonable low approximation limit for color adjustment

this prevents within s&h some of the typical artifacts in highly saturated blues
(black pixels). the original threshold for low approximation was insanely high.
legacy history stacks are processed like with the old value in order to keep results
consistent. you need to reset the module in these cases to take advantage of the improvement.
(cherry picked from commit 2522d40e4350f40ed6ba2df1d7f283e6ac57b1fe)
Update French translation.
(cherry picked from commit 0b2aa1ce45dc038bd31c464d6f993fbcd5cdec19)