Allow to change unit price from the sale point line
We should allow changing the unit price of a sale point line. Some examples:
- Customer ask for reduction due to some minor design flaw that does not affect the usability (scratches on laptop / stain clothes).
- When removing some key ingredients in a restaurant.
- When promoting that if you find a low price on the competitors, you will match.
I also see a comment 'readonly': True, # Allow client to sent value
which I don't understand if it's a TODO or something I should check or do different on the client.