Fix EZP-23617: Add subtree and class filters on eZ Find autocomplete

Replaces https://github.com/ezsystems/ezfind/pull/106

Note that this adds the capability to eZ Find. The various templates (object relations, Online Editor) still need to be updated if they want to use it. (Example: https://github.com/ezsystems/ezpublish-legacy/pull/620)
Nitpick:
It seems $args[3] **must** be a non-empty array for the following logic, so maybe:
`if ( isset( $args[3] ) && is_array( $args[3] ) && !empty( $args[3] ) )` ?