This commit moves the logic associated with the composer placeholder for two reasons: - move business logic from components to models - allow to easily override the placeholder in other module. This is uselfull for the feature allowing to leave comments on a spreadsheet. Task 2638411 (spreadsheet comments) Task 2793280 closes odoo/odoo#86381 Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>