Split the getting of the skinr ui edit form defaults out into their own function

Instead of #1942418: Option to have the skinr form integrated with the entity form it relates to, it would be good to split the default value getting out of skinr_ui_edit() into its own function.

This makes it easier to more cleanly replicate the skinr_ui_edit() form elsewhere.

For example, I am form altering my node form to add skinr_ui_edit() to it in a collapsible fieldset.
To do that I need to set the equivalent of:

<?php
    $form_state['values']['skinr_settings'] = $skinr_defaults;
?>

However in the current state if I call drupal_get_form('skinr_ui_form') I can't have that.

I propose a system where I can do this:

<?php
    $skinr_defaults = skinr_ui_form_get_defaults($module, $element);
    $skinr_form = drupal_get_form('skinr_ui_form', array('module' => $module, 'element' => $element), $skinr_defaults);
?>