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.
