L10n NL Account Tax UNECE

This module adds no new screens or menus. It works behind the scenes on the Dutch tax setup.

• It sets the UNECE tax type to "VAT" and the matching UNECE tax category on the standard Dutch tax templates (the blueprints used when a company installs the Dutch chart of accounts).
• Categories are assigned per tax: zero-rated and exempt sales get the zero/exempt categories, standard and reduced Dutch rates get the standard-rate category, reverse-charge taxes get the reverse-charge category, EU intra-community supplies get the intra-community category, and exports outside the EU get the export category.
• When the module is installed, a one-time action runs for every existing company: it looks at the company's taxes, finds the template each tax came from, and copies the UNECE type and category onto the live tax records.
• Because the values are also stored on the templates, any new company created later with the Dutch chart of accounts automatically gets correctly tagged taxes, which is useful in multi-company setups.

It touches account.tax.template, account.tax and res.company. It is installed automatically when both the Dutch localization and the UNECE tax module are present.