[16.0][IMP] sale_global_discount: refactor _compute_amounts

[trisdoan]

Context

In simple case (tax is not included in price), flow of the module as follow:

• Let tax is computed as usual, as odoo native supports
• Afterwards, checking if global discount is applied on Sale order:
  1. If not, only add amt_untaxed_before_global_discount and amt_total_before_global_discount,
  2. If yes, precompute tax

A example of Sale order is below:

• Tax is not included in price
• With 20% global discount

Product Name	Quantity	Unit Price	SubTotal
Example Product	2	$75	$150

Untaxed Amt
$120

Tax
$18.0

Total
$138.0

=> So tax is recalculated based on Final price (after global discount)

• Before this change, if enabling included-in-price tax, it breaks the intended flow as it triggers the part 'handling price include' again with the new price

This change

• pass param: handle_price_include=False because it was handle in super(), by default handle_price_include=True . Now, as we recalculate tax to apply global discount, there is no need to handle price included or not.

• With above example, now it is:
  - but this time, Tax is included in price

Product Name	Quantity	Unit Price	SubTotal
Example Product	2	$75	$130.43

Untaxed Amt
$104.34

Tax
$15.65

Total
$119.9900

• Add a check to only recompute tax when global discount is applied on the sale order. Before this change, it results in rounding issue. It happens when products are configured as tax-included in price. For example: