fix(sidebar): set next/prev properly on deep nested tree

[shcheklein]

Fixes #200

The issue was in not going deep in enough in children hierarchy to set the "previous" item pointer. It was setting it only to the next level of children:

- first item
  - nested item <--- it would always pick this as a previous while processing the `second item` below
     - nested nested item
        - - nested nested nested item <--- we should go down the tree to the bottom to find the deepest child
- second item

[yathomasi]

Thanks @shcheklein. Looks good to me.