Sanatize sorting state of folders

The sorting state of folders in Hippo CMS is subtle and tricky to grasp. This confuses users and makes the implementation unnecessary complex.

Currently, we have two types of folders: ordered and unordered. In ordered folders, the order of children matters and can be changed by the user. The default sort state of these folders is "stored order". In unordered folders, the children cannot be reordered. The CMS shows children of an unordered folders default sorted ascending name.

The sort order of a folder can be changes by clicking a column header in the list view.

Problem 1: the sort state is kept irregularly. The sort state of ordered folders is kept when switching to another ordered folder, but reset when switching to an unordered folder. The sort state of an unordered folder is always reset to the default when it is selected.

Problem 2: the initial sort state of ordered folders (ascending by "stored order") is hard to notice, since the column header does not show a sort label anywhere.

First idea of arthur: add a column that shows the index, so we can sort on this column by default and keep state among ordered/unordered folders. This is also more clear to the user, as right now sorting on the stored order is not reflected in the UI (no column is showing an arrow, even though there is very specific ordering begin done).

Second idea of arthur: ditch the unordered folders altogether. It does not really add anything, apart from making this hard to solve and hard to grasp by a user.