1) Do not overwrite lines description if the line was not modified
Fixes#78833
2) Correctly recompute prices on quantity change.
When no new line was added to the SO through the matrix, the prices were
not recomputed, even if the qty was modified for some products.
The prices have to be recomputed in this case since pricelist provide
rules based on minimum qty.
X-original-commit: 4f819438aafead57d6bc79e03f9a7980c6420783
Part-of: odoo/odoo#78971
Victor Feyens
e9c487dba8