This commit aims at removing the over-complexity of intrastat modules
while increasing simplicity/usability for users.

- Move default intrastat transaction from res.company to account.fiscal.position to add su
pport for B2C (and not just B2B)
- improve usability: auto-generate declaration lines and XML export when
going from draft to done. Auto-delete declaration lines and XML export
when going from done to draft (and add confirmation pop-up).
- declaration lines are now readonly. Only computation lines can be created/edited manuall
y
- add field region_code on computation lines and declaration lines.
Remove region_id on declaration lines. This change allows big
simplification in some localization modules such as
l10n_fr_intrastat_product.
- simplify Brexit implementation. Northern Ireland is important, but we
can't afford to have so many lines of code and add a field on
product.template (origin_state_id) for a territory of 1.9 million
inhabitants! This is too costly to maintain and too complex for users.
- improve default visibility of fields when reporting_level = 'standard'
- add support for weight calculation from uom categories other than
units and weight, supposing that the 'weight' field on product.template
is the weight per uom of the product
- add EU companies from several different countries in demo data with valid VAT numbers

…ND company

Odoo uses, by order of priority:
1) intrastat transaction of the invoice
2) default intrastat transaction of the fiscal position of the invoice
3) default intrastat transaction of the company

…ning pop-up

This is completely new implementation of the messages that are displayed
in the warning pop-up:
- no double messages
- messages are grouped by section and record
- nice HTML display insead of raw text

Make tests work

When manually creating a computation line, auto-set vat from partner_id
(partner_id is not a related field any more)
Add method _generate_xml()