Allow dynamic #process and #pre_render hooks for elements

Due to a core bug/feature, if a form element specifies a #process or #pre_render property in the render array, it's default callbacks are not added from the default element info (@see #914792: Custom element properties entirely override default element info properties).

The "solution" is to instead do the following:

$form['item'] = array(
  '#type' => 'fieldset',
  '#pre_render' => array_merge(array('bootstrap_subtheme_pre_render_fieldset'), element_info_property('fieldset', '#pre_render', array())),
);

This is, in fact, very cumbersome to do every single time. Instead, the alternative is to do a proper alter in hook_element_info_alter():

function bootstrap_subtheme_element_info_alter(&$info) {
  $info['fieldset']['#pre_render'][] = 'bootstrap_subtheme_pre_render_fieldset';
}

This too is very cumbersome. It would be much better if there were a proper "hook" system in place to automatically look for these functions and then add them in the default element info. Of course, since this would cause bootstrap_element_info_alter()to become heavy/time consuming, it will need to be cached.

There will also need to be a way to skip elements from invoking these hooks if necessary.