Add a Disabled visual state for UIControl components and ItemVisuals

[adgray1]

Logging request per this discussion!

[wrymn]

Yeah this, I just installed NOVA to try it out, if we can use it for our next game project in the studio, and this was one of the first things that I tried, and it was missing to my surprise

[NGenesis]

It's baffling that this isn't already part of the framework since it's required for any kind of UI development.

[marc-antoine-girard]

It sounds like this could be implemented in a single day. We added it in our project on some controls because of course we need disabled controls. I'm sure I could provide the code, but I'm wondering if it hasn't been implemented in Nova because you are thinking about changing the codebase for the samples or something...