Before, clicking "new problem" (etc) would create a new block, then launch the editor. Now it launches the editor and then creates the new block only on save. This makes the "Cancel" button work as expected. Only affects libraries so far, not courses.
Creates the AdvancedEditor to support editors like Drag and Drop, openresponse, poll, survey, and other advanced editors.
- AdvancedEditor created to call studio_view of the block
- Update LibraryBlock to support any view (and use studio_view in AdvancedEditor)
Intercept xblock-event message to close the Advanced editor on cancel or save
