Improve row & section hiding logic

[zoul]

The current version is too hacky. Main problem is that the table view has really no concept of hiding cells or sections, it can only delete and insert them. While the two cases (hide/delete, insert/show) are quite close to each other, the table view insists on knowing precisely when a row or a section is inserted/removed (we can’t simply reload the table for UX reasons) and we have to be careful around the indexes, as they change while something is being added or removed.