Allow sub selects to be of array nature.

[discordier]

Now we support to define subselects as:

'metasubselectpalettes' => array
    (
        'field1' => array
        (
            'value1' => array
            (
                'field2' => array(
                    'field3',
                    'field4',
                ),
            ),
        ),
    ),

This will then place "field3" and "field4" after "field2" when "field1" holds value "value1".

We still have to decive if we also want to allow the after and before syntax somehow.