LPS-44508 please read comments - new taglib for staging

Hey Brian,

The staging UI is getting more and more complex, and it's getting hard to maintain the code and develop new UI portions on the top of it. I've created a taglib for the staging UI pieces, and I've moved an already existing tag (liferay-ui:staging) to it, but more to come. This pull contains only the new taglib and the refactoring of the existing.

reviewed by @KocsisDaniel

Thanks,

Máté