only use alpha channel if it exists
[FIX] api: avoid to return all fields *2many in onchanges

When an onchange returns a change in a 2many field line (a '1' tuple, update), avoid to return all fields of the *2many field but only the altered field.

Otherwise, the web client regard all the fields of the 2many as dirty, and try to write on all fields (even if the value is the same, thus)

opw-615062
[IMP] Remove unnecessary refresh statements because of new api
Revert "[IMP] Remove unnecessary refresh statements because of new api"

This reverts commit b213438176b16e96686d94063a3cdf6dc5c49618.
[fix] portal: typo in description

closes #3685
[FIX] ir_model: on a custom model, _rec_name should be 'x_name' if it exists
[IMP] doc: indexify index

* opt-in toc navification
* unhide index toctree, it's now back to a list of documents
* remove text thing
[FIX] tests: make sure that a failed tests does not leave the environment dirty

When a failure occurs, or when exiting an assertRaises(), the environment
should not contain fields to recompute.
[IMP] doc: more indexy index
[IMP] doc: move old API sections to orm toplevel

also move porting checklist/doc/thing to bottom of document, it's mostly
for seasoned or internal developers
[IMP] doc: try to make some bits of the porting guide less confusing
[FIX] tests: make self.assertRaises() return the expected object
[IMP] doc: rename WS page, lift to top-level
[IMP] doc: wording of method-calling introduction
[IMP] doc: have ws page take whole page width

Doc pages used container() which is width-limited, works acceptably for
single-column content but WS page takes 2 columns with code, having as
much space as possible is a much better idea.

Mayhaps it should even use lg-column(2) instead of md-column(3)?
[IMP] doc: improve index, WS page
[FIX] website_forum - replace texttext by select2 for the tags fields
[FIX] website_forum: add fake tour for people waiting the tour. To remove in master. Without it, localstorage loop to search this unexisting tour after the install...
[FIX] doc: latex building

* add latex support for exercise admonition
* latex freezes/crashes on {HEAVY WIDE-HEADED RIGHTWARDS ARROW} so
  replace them by more manual arrows
* add some preamble configuration, may not even be necessary
* generate WS setup code only in HTML output. WS doc in latex still
  isn't great as it displays all 4 languages one after the other,
  ideally they should be tagged or something, so only one language at a
  time is generated in non-HTML outputs
[FIX] doc: double period in footer