[FIX] mrp: Avoid creating new line when changing qty to produce

When we change quantity to produce in mrp, all component lines are
removed and new line with the updated amount to product are created.
The problem appears when we change the quantity to produce and directly
click on a component line, because we open the line that will be
deleted. So if we change the quantity for this component manually,
this suppressed line will be added with the new value. And the computed
line will be also be added. So the line for one component will be
present twice (one with computed value and one with manual entry value).

To avoid this problem, when we change the quantity to produce, we update
information of each line (instead of deleting and creating new one).
So if we click on a component line just after changing the quantity,
we open the updated line (and not de deleted one).