(Font)Awesome tree icons =)

Added custom TreeNodeUI provider to allow fontawesome icons in the tree.

Default element icon classes are defined in `modElementGetNodesProcessor::getNodeIcon()`

Icon classes for all elements can be overridden with system settings in the form of `mgr_tree_icon_[elementName]`

Finally utilised the modTemplate::icon field! Yaaaaaaay! Set this field to a fontAwesome icon class and all resources using this template will use that icon instead :sunglasses: 

Still to discuss/do: 
* File tree - how to handle file-type icons
* Add different resource icons for resource types (and allow custom resource classes to specify their own perhaps?)
* Possibly find a system setting based way of changing icon colours? Might be overkill