[acsone-addons][add] Mail Html Wigedt Template to manage placeholder

Mail HTML Widget Template
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This module allow to immediately manage placeholder from the mail composer
This tools is available on each html widget of Odoo

* Primary dropdownlist is filled with expected fields of the object associated
    to the template.
* When choosing a field in a dropdownlist, two possibilities:
    * If the selected field is a relation to another object, an additional
        dropdownlist filled with the expected fields of this object is append
        to the toolbar (this generic mechanism follows relations between objects
        through the data model and is intended for an undefined number of
        levels)
    * Otherwise the placeholder is inserted into textarea