Create "head_title" page variable using a theme hook

The "head_title" page variable is created in panels_everywhere_page_preprocess_elements():

function panels_everywhere_theme_registry_alter($registry) {
...
  // @todo add a settings form to control the following parts of this:
  // separator
  // use site name in title
  // use slogan in title
  // Construct page title

  $head_title = array();
  $page_title = drupal_get_title();
  if ($page_title) {
    $head_title[] = strip_tags($page_title);
  }

  if (variable_get('panels_everywhere_head_title_include_name', TRUE)) {
    $head_title[] = strip_tags(variable_get('site_name', 'Drupal'));
  }

  if (!$page_title && variable_get('panels_everywhere_head_title_include_slogan', TRUE)) {
    $head_title[] = strip_tags(variable_get('site_slogan', ''));
  }

  $variables['head_title']        = implode(variable_get('panels_everywhere_head_title_separator', ' | '), $head_title);
...

1. There is a little bug: if there is no slogan, the title ends up being "My site name | ", it should just be "My site name".
2. I suggest moving this to a theme hook so that we can just override that hook to make customization (then there is no need to provide settings form as the todo comment suggest). Right now, to customize, we need to add code in our own page preprocess function and erase what panels_everywhere has done and re-do over.

This can be simplified to:

function panels_everywhere_theme_registry_alter($registry) {
...
  $variables['head_title']        = theme('head_title');
...
}


function theme_head_title() {
  $head_title = array();
  $page_title = drupal_get_title();
  if ($page_title) {
    $head_title[] = strip_tags($page_title);
  }

  $head_title[] = strip_tags(variable_get('site_name', 'Drupal'));

  if (!$page_title && $slogan = variable_get('site_slogan', FALSE)) {
    $head_title[] = strip_tags($slogan);
  }

  return implode(' | ', $head_title);
}