Separate the concept of "tax account" and "tax category"/"tax rate"

[ehuelsmann]

The idea being that a "tax account" is an account on which the taxes of a category (VAT/sales tax being a category) are posted. The tax category/tax rate has multiple instances, each with a different rate. These need not be posted each on their own account (although they could, when each rate gets its own category).

In order to make this work, the eca_tax table should not bind an eca to an account, but instead to a tax (tax instance/tax rate).

Another thought - which I need to test with @freelock, @ylavoie and @sbts - is that the customer gets tied to one or more tax categories ("Dutch VAT"?) where each category has multiple rates that get tied to the parts/services/etc. Is this over-engineered?