Currently deleting a category with slides is not supported as it should be voided before being unlinked. This commit allows to unlink a category by moving all its slides in uncategorized section. This require a small reordering of sequences as category computation is based on sequence of slides in a given channel. Task ID 2276559 Prepares Task ID 2093671 PR #52917 Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>