[IMP] web_editor, website: introduce foldable zones in the editor UI

Now any set of widgets and rows can be placed in a we-collapse element
so that those elements can be foldable. The first element inside (or
the mentioned "string" on the we-collapse element itself) is used as the
always shown element (with a toggler on its left to toggle the
visibility of the other items).

we-collapse inside other we-collapse is supported as well.

Limitations / Other infos:

- The use of '⌙' and   characters is not automatic (yet?).
  Sometimes it actually make sense to use them without a we-collapse or
  to not use them in a we-collapse.

- The use of we-collapse is limited to widgets inside the same option.
  So unfortunately, there is no current way to add all bg image options
  in a folable section as those contain many different options. Maybe
  in the future.

Part of https://github.com/odoo/odoo/pull/56350



closes odoo/odoo#56350

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