Since [1] form fields cannot be moved outside of their form and since [2] table of contents cannot be nested within another table of content. Those were implemented with specific references to website classes within web_editor. This commit introduces declarative mechanisms for both situations: - `data-drop-lock-within`: prevents dropping outside the closest parent that matches the specified selector. - `data-drop-exclude-ancestor`: disallows dropping within a parent that matches the specified selector. [1]: https://github.com/odoo/odoo/commit/638d0e875dcb05191fe833d2f889235891dfb46a [2]: https://github.com/odoo/odoo/commit/55339cd6a7b7916184893c0fe27b5483683a047a task-3131384 closes odoo/odoo#130477 Signed-off-by: Guillaume Dieleman (gdi) <gdi@odoo.com>