Add some admin theme callbacks and make them explicitly admin_xxx for consistency

with the callbacks in the xxx_block helpers.  So in the theme we have:

admin.html.php:
  $theme->admin_page_bottom()

then in the helpers:

core_block.php:
  function admin_page_bottom() { }

core/libraries/Admin_View.php

@@ -65,9 +65,11 @@ class Admin_View_Core extends View {
    */
   public function __call($function, $args) {
     switch ($function) {
-    case "dashboard_blocks":
-    case "sidebar_blocks":
-      $function = "admin_$function";
+    case "admin_dashboard_blocks":
+    case "admin_footer":
+    case "admin_page_bottom":
+    case "admin_page_top":
+    case "admin_sidebar_blocks":
       $blocks = array();
       foreach (module::installed() as $module) {
         $helper_class = "{$module->name}_block";