LPS-40537 Themes are not exported anymore but referenced to be validated if necessary during import

Hey Brian,

I've added a wrapper object for Theme to fit in the staging so it could act like a StagedModel. I haven't modified the Theme interface or the ThemeImpl itself I wanted to separate this special staging logic being added for validation from the original behavior. This could be a good pattern in the future maybe for similar entities are not Classed/StagedModels

Wasn't sure about the name ThemeStagingWrapper, if you think something else would be good for it, just let me know and I'll rename it accordingly (with the language keys and other stuff)

Thanks,

Máté

reviewed by @KocsisDaniel