Filtered navigation improvements (multivalue, iconised)

We require some improvement to the YUM product/sku manager to better manage multi value attributes. There are some "behind the scenes" functions that need to be addressed in order to provide duplicate detection for single value attributes and improvements to the UI (there are some glitches when attributes are updated)

Whilst working on this we need a mechanism to represent values as icons. For example colour attribute is better represented by a colour palette rather than enumeration of colour names. Other use cases can be fabric features that are better known through icons (e.g. machine washable, dry-cleaning only, leather)