Description of the issue/feature this PR addresses:

Accessibility improvements forbids the use of the syntax
`<i class="fa fa-check"/> Some text`
to create a labelled icon. But, by this, some fonts are changed.

Desired behavior after PR is merged:
The old syntax can be used.

Today, Odoo is really tricky to use without seeing the screen, it must be improved to be usable.

This PR forbid to use labels without a "for" attribute, add some title, rule and aria attributes in HTML. With that, Odoo will be fully usable with a screen reader.


* [IMP] Labels must have a for attribute. Improve accessibility.
* [IMP] Better error message when trying to read a missing cached value
* [FIX] Add some aria-label and title attributes for screen readers.
* [FIX] Template name is not included in the error message in case of SyntaxError in QWeb
* [FIX] Improve the Tour failed at step error message to be more explicit.
* [IMP] Add aria-labels
* [FIX] Add missing aria-label on failing test
* [IMP] aria-hidden means hidden. Fix all bad aria-hidden and hide aria-hidden for all.
* [IMP] Color names on kanban views and many2many tags
* [IMP] Add some checks on views for accessibility.
* [IMP] Add `alt` attribute on `img` tags.
* [IMP] Add aria-label and title on non-described icons
* [IMP] Add button role to widgets with btn class
* [IMP] Translate aria and formatted attributes.
* [IMP] Remove wrong aria-labelledby
* [IMP] Add menu role on dropdowns
* [IMP] Buttons must be focusable
* [IMP] Add aria attributes on progress bars
* [IMP] Improve accessibility of basic widgets
* [IMP] Change main layout to more semantic tags
* [IMP] Add menuitem role when missing
* [IMP] Remove wrong role='presentation'
* [IMP] Improve accessibility of tab panels
* [IMP] Add aria-invalid on invalid fields
* [IMP] Add aria-sort on ordered columns
* [IMP] Add role on alerts
* [IMP] Use dialog role, header, main and footer tags for modals
* [IMP] Add labels on o_status
* [IMP] Improve accessibility of kanban view with feeds and articles
* [IMP] Add alerts in case of new messages
* [IMP] Add widget, navigation or img role to aria-labelled items