[IMP] web_editor, *: add new zone in left panel to handle invisible DOM

* website, website_mass_mailing

We only have one invisible snippet (newsletter popup) which is handled
by adding a yellow alert in the DOM in edit mode to be able to click
somewhere to open the snippet options.

As we will add some new invisible snippet (generic popup, cookies bar,
and probably more) this needed to be improved.

Now, any snippet having the `o_snippet_invisible` class will have an
entry in the left panel in a dedicated zone to edit itself. This also
allows to get rid of the yellow alert, making sure that "what you see
is what you get" in the editor zone.

task-2150966

closes odoo/odoo#41420

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>