Add Multi-channel content tree

Description

A Multi-channel Content gadget was added to let you view and work with folders, blocks, and images; pages are not usable in this gadget. You can remove the Navigation view to make Multi-Channel Content the primary gadget, and restore Navigation view by selecting Settings (gear icon) > Add gadgets. For more information, see Headless mode: editing multi-channel Content in Optimizely in the user guide and Headless mode: multi-channel content in Optimizely in the developer guide.