Allow modules to assign region to form elements

Hi,
I wanted to put some "subfields" in the right region but found it was not possible with the current code (maybe I missed that option ). But in fact it took 3 lines of code to make it possible ( was 2 before ), and I think this option can become quiet useful.

With that patch it's possible to do things like that:

function mycustom_module_form_alter(&$form, &$form_state, $form_id) {
...
$form["my_extra_field"]['#placement'] = array(
    'region' => 'right',
    'weight' => 3,
    'has_required' => FALSE,
    'hidden' => FALSE,
  );
}

Things that become possible in hook_form_alter() with this:

1. Assign region to form elements
2. Move fields depending of the user role
3. Move subfields :)
4. ...