Allow modules to register icons

@immortius, I know NUI will allow us to register icons as assets, but we could use something right now to get a number of modules working again.

@MarcinSc, does this APIi work for you in the short term?  I know you mentioned you needed the same support.

I'd also be willing to rework this as an event / system, or as a CoreRegistry service with an interface, but since we know this is only a short term workaround, I don't know that it's necessary.