Add `a_pred` to `new_menu_nested`

For attribute filters, don't include attributes without `sig_key`, i.e.,
composite attributes. For order_by and attribute selection, do include
those.
Fix disabling of nested attribute menu entries

Need to use the `label` of the attribute, not the `html` of its menu
entry!

Fix `attr_select.prefill` call of `toggle` (needs to pass `af.label`,
not `choice`).
Add polymorphic attributes to filter menu