Add priority field to class icons.

To determine which icon to used when multiple possibilities.

For now primarily using this to choose the Singleton icon over
Component.
Move class_icon providers filtering out of TagRegistry.

Add a generic #tags method to TagRegistry and do the filtering in
Web::ClassIcons instead.
Use class << self pattern in Web::ClassIcons.

Instead of prefixing each static method with "self.", use this
Ruby trick to place all these methods into the 'eigenclass'.