This will help us eliminate the use of file names for icons and instead move to a CSS class spec type model.

I think this should be done by adding a new interface in the icons-set module (located in the icon-shim plugin repo https://github.com/jenkinsci/icon-shim-plugin). This will allow plugins maintain backward compatibility with older versions of Jenkins core.

e.g.

 
interface IconSpec {
    String getIconClassName();
}

Then... plugin extension points that implement/extend Action/ManagementLink can also implement this interface.

Then... jelly scripts that use 'iconFileName' (e.g. core/src/main/resources/jenkins/model/Jenkins/manage.jelly) can be changed to use 'iconClassName', falling back to 'iconFileName' if 'iconClassName' not available.