[ADD] doc: document domain operators, fix typos

* Add description for all allowed operators.
* Improve wording of domain documentation and fix spelling.
[FIX] models: in _add_field(), set the field as an attr before setting it up

In the case of custom fields, the field's parameters were set up without the
field being present in the class hierarchy.  Because of this, the parameter
inheritance mechanism was missing the field itself.  As a consequence, custom
selection fields ended up without selection, for instance :-/
[FIX] product: redo forward port 2832d2d for fix 12eb157 lost with 43977de
[FIX] product: use new api to retrieve product standard price as SUPERUSER

As it looks like self is a product.template, and product can be a product.product. Using the new api avoid to have this problem.
[FIX] email_template: do not try to add user signature if none set

append_content_to_html expects two strings. If the user has no signature, it returns user.signature returns False
[FIX] point_of_sale: fixed display of untaxed amount in sales lines report
[FIX] point_of_sale: fixed display of untaxed amount in sales lines report
[MERGE] forward port of branch 7.0 up to 75d3ea6
[FIX] website_quote: compare untaxed amounts in case of discount
[MERGE] forward port of branch saas-3 up to db75994
Merge pull request #3001 from Bubbles-IT/8.0-fix-pos-tax-label

[FIX] point_of_sale: printing of tax name on hardware was missing name
[FIX] models: do not look up the registry class when building cls._fields
[FIX] implementation of t-call's 0-index in qweb-js

Had expected regularity, had not noticed the Python version is really an
ignoble hack where the string "0" (exactly) is interpreted in a specific
manner when used in t-raw and format strings, *and nowhere else*.

Had special-cased the expression everywhere which broke e.g.
<t-set="xxx" t-value="0"/>. Reproduce Python behavior more closely,
and strongly consider swapping out both for something different e.g.
[[content]] or something like that, which doesn't cause problems in
Python but doesn't have to be so specially cased either.
[FIX] website: editor: preserve label and color for link buttons targeting a page

When selecting a page target rather than a static
URL the style and label information were ignored/lost.
[FIX] web: push render value deferred only if the field is in a form view
[FIX] mail_thread: allow to pass mail_track_log_only in the context to remove the subtype of the notification. By this way, no mails will be sent because partners_to_notify will be empty.
[FIX] web: even if there is no view, the field has to be rendered

This is related to ÿ07f3f4d9206d4c1a3e484eb89f09d2258977a
[FIX] product: correct name_get for products with resellers.

Error introduced during forward-port commit 14f3085
[FIX] Currency_id on purchase requisition + move created from procurement should go to location of procurement not of rule (procurement location can be child of rule location.  In handy for customer locations)

[FIX] Other elements of purchase of purchase requisition

[IMP] Picking type on purchase requisition

[FIX] Location of purchase
Merge pull request #3081 from odoo-dev/8.0-wmsprocmoveloc-jco

[FIX] Currency_id on purchase requisition + move created from procuremen...