Tabs: Finish API Redesign

There are a few things that we still haven't fully worked out. We plan on replacing the fx option with show and hide options for consistency with other widgets, but are still working through an open issue of how to support effects across plugins. We would also like to remove the load method but we need to verify that it can be built as an extension. Until we get into the new implementation, we won't know if this is possible; if it's not, the load method will remain in the plugin.