[FIX] core: inherit translations better when upgrade

[HydrionBurst]

Before:

The typofix feature treats terms in the old and new values with similar text
content as the same term, migrating the translations of the old term to the new
term.

For example

The old value has the mapping:
'Draft': 'Brouillon'

The new value contains the term:
'<span invisible="name or name_placeholder or quick_edit_mode">Draft</span>'

Since the old term and the new term share the same text content, 'Draft', after
write, the new term reuses the old translation of 'Draft'. However, the
translation 'Brouillon' is always visible, unlike its en_US counterpart.

This behavior is acceptable in non-upgrade mode because the user writes the
en_US value and is responsible for verifying translations afterward. However, it
is problematic during upgrades because users cannot easily identify which
records have changed and need to be rechecked.

After:

The translation inheritance behavior can be described as below
Translations can be inherited after write from old terms to new terms which
share the very close text contents

1. when write in production mode, text contents for translation terms are more
   important than the HTML/XML structures of them, and the old term translations
   should be remained as much as possible. Because
   • the writing user is responsible to recheck all translations after write.
   • it is easier for the writing user to copy technical HTML/XML structures
     than translate text contents for a language they may not know.
   • the feature can also be used as typofix when the only small diff is the
     text content
2. when write in upgrade time, the HTML/XML structure is more important than
   the text content, and the new term structure should be remained as much as
   possible. Because
   • HTML/XML structures might be changed a lot after upgrade, which may
     contain behavior relevant diff (e.g. invisible), even if text contents
     are not changed.
   • users have no idea which records' values are changed during upgrade and
     are hard to recheck their translations.
   • new terms are highly likely to be correctly translated in the latest po
     files which will be imported during upgrade.
   • the typofix feature can still be remained when the only small diff is the
     text content

Based on the above feature analysis, we use the below new strategy

1. translations can be inherited only if the old source term and the new source
   term share the same HTML/XML structure
2. translations can be inherited only if the old translation term and the new
   source term share the same HTML/XML structure
3. when translations are inherited, MODIFIER_ATTRS will be synchronized with
   the new source term, other attributes will be copied from the source term if
   available.

[robodoo]

[aj-fuentes]

Can we also add something like

        # order of terms was inverted
        archf = '<form>%s<div>%s</div></form>'
        terms_en = ('<i>Draft</i>', '<span invisible="1">Draft</span>')
        terms_fr = ('<i>Brouillon</i>', '<span invisible="1">Brouillon</span>')
        view = self.create_view(archf, terms_en, en_US=terms_en, fr_FR=terms_fr)

        archf = '<form>%s<div>%s</div></form>'
        terms_en = ('<span invisible="1">Draft</span>', '<i>Draft</i>')
        terms_fr = ('<span invisible="1">Brouillon</span>', '<i>Brouillon</i>')
        view.with_context(install_mode=True).write({'arch_db': archf % terms_en})

And perhaps the same term-inverting tests but with a pure text element as well.

[aj-fuentes]

LGTM. Nice work!