Add option :hidden: to ObjectDescription.  Currently, this option has no
effect except being stored as attribute in the resulting node.

Do not produce any output only a target node if option :hidden: is
given.  In this case all nodes representing this ObjectDescription are
replaced by a single target where the target gets assigned all ids of
the replaced nodes.

Record the source information (class attributes source and line) when
replacing an object description node with a target node.

The collect_ids() function uses the its second parameter ``ids:
List[str]`` as in- and output parameter and always returns None.
Rewrite the function to become pure and side effect free: return the
list of ids as the return value not as an output parameter.

An index directive creates a index node followed by a target node.  Two
consecutive index directives::

.. index:: first
.. index:: second

create index, target, index, target, i.e. a mixture.  The interspersed
index nodes prevent other transformations, e.g. PropagateTargets to
properly work on the target nodes.

Apply a transformation which reorders mixed and consecutive index and
target nodes such that first all index nodes are before all target
nodes:

Given the following document as input:

    <document>
        <target ids="id1" ...>
        <index entries="...1...">
        <target ids="id2" ...>
        <target ids="id3" ...>
        <index entries="...2...">
        <target ids="id4" ...>

The transformed result will be:

    <document>
        <index entries="...1...">
        <index entries="...2...">
        <target ids="id1" ...>
        <target ids="id2" ...>
        <target ids="id3" ...>
        <target ids="id4" ...>