[IMP] pos_discount: usability improvements regarding discount setup

Before this, setting up the discount on a pos_config was a bit
confusing. Although iface_discount was enabled by default on the
pos_config, the discount button would not show up in the actual POS. The
reason for this is that the discount button only shows up when a
discount product has been set, and there is no default for that.

So in order to make things a bit clearer the following has been done:
- Group discount related options together
- Only show discount product and default percentage field when using discount
- Rename 'Discount percentage' -> 'Default percentage'
- Default iface_discount to False (because a discount_product_id has to
  be manually set anyway)
- Make discount_product_id required when discount is enabled
- Only allow discount_product_id candidates that have available_in_pos set,
  because the POS frontend only loads those products

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