navigation MDL-25596 Improvements for the navigation blocks JS

The biggest change is that the navigation block has been converted to a proper YUI module.
The following are the other changes made at the same time:
 * A loading icon is displayed when a branch is being loaded by AJAX.
 * Fixed a bug where you could trigger multiple AJAX requests by rapidly clicking an unloaded branch.
 * Fixed a bug where empty branches weren't being marked as such after a successful AJAX load.
 * When docked the width of the blocks dock panel is now inspected an increased if required to try avoid horizontal scrolling.
 * Removed the no longer needed inclusion of the YUI2 dom library from the navigation and settings block.
 * Expandable nodes are now passed as JS data allowing the navigation JS to be initialised through block_navigation::get_required_javascript.
 * AJAX is now focused around the branch in question rather than the tree in general.
 * Expansion of branches is now delegated to the tree rather than being an individual event on all branches.
 * Tidied up the code in general removing unneeded-unused parameters.

lib/navigationlib.php

@@ -540,7 +540,7 @@ class navigation_node implements renderable {
             if ($child->nodetype == self::NODETYPE_BRANCH && $child->children->count()==0 && $child->display) {
                 $child->id = 'expandable_branch_'.(count($expandable)+1);
                 $this->add_class('canexpand');
-                $expandable[] = array('id'=>$child->id,'branchid'=>$child->key,'type'=>$child->type);
+                $expandable[] = array('id'=>$child->id,'key'=>$child->key,'type'=>$child->type);
             }
             $child->find_expandable($expandable);
         }
@@ -3699,7 +3699,7 @@ class navigation_json {
      */
     public function set_expandable($expandable) {
         foreach ($expandable as $node) {
-            $this->expandable[$node['branchid'].':'.$node['type']] = $node;
+            $this->expandable[$node['key'].':'.$node['type']] = $node;
         }
     }
     /**