In this commit, all usages of env._t() are replaced by _t().
In templates files, env._t() didn't work because terms used
in attributes where not extracted into the translation files.
Only string are exported from .xml files to translation files.
So, to make it works, we set a variable that is then used
in attributes.
For example :
<t t-set="string_to_translate">String to translate</t>
<Dialog title="string_to_translate>...</Dialog>
task-3292454
closesodoo/odoo#131390
Related: odoo/enterprise#45631
Signed-off-by: Michaël Mattiello (mcm) <mcm@odoo.com>