Move the widget_x2many tour to web and call it from test_new_api as it needs
to be overrided for the new design, and it requires models and data from
test_new_api.

Clean other unnecessary stuff from web_tests, and thus remove the addon.

Refactoring of the DOM structure and classnames of the list view's action
buttons (in the control panel), for new design compatibility. This refactoring
was required as some addons shared between both editions (e.g. base_import)
define their own buttons and extend the ListView.buttons template to insert
them.

Many2One fields were broken in views opened by default in edit mode, since rev.
a0ddaf8. The autocomplete widget was appended to the DOM at
'.oe_application > *:visible:last', but since the mentionned rev., the views
are rendered in fragments detached from the DOM, and once they are ready, the
old content is detached and the new one is attached instead. It means that by
appending the autocomplete widget within '.oe_application > *:visible:last',
it is detached from the DOM with the previous content, and Many2One fields can
thus not work.

This rev. simply removes the appendTo option of autocomplete, so that the
widget is directly appended to the body.

Also updates a tour's selector matching an autocomplete ul as it isn't inside
the .modal anymore.

Use a fontawesome icon instead of no more supported mnmliconsRegular.

The widget_x2many tour uses the discussion model to test one2many
and many2many widgets. The tour clicks on an item of the menu which
opens the discussion model with the discussion_form_2 view (in create mode).
Then, it clicks on the view_switcher to display the list view, and clicks
on an item of the list to open a previously created record.

In the new design, there is no more view_switcher in form view. This flow
is thus unfeasible. This solution changes the menu action so that it
directly opens the model with the list view, on which we can click on the
record to display in form view.

This rev. generalizes the notions of FieldOne2Many to FieldX2Many and of One2ManyViewManager
to X2ManyViewManager. The same logic is now applied both for One2Many and Many2Many fields.
This allows Many2Many list and kanban views to have a pager, as the pager is instanciated
by the view, on demand of the ViewManager.

Adapt web_tests to the new many2manytag included in the tour widget_x2many

Update css classnames used in web.tour.js and add them to corresponding
elements in web