Folder pinning

[kirillt]

Scenario:

1. User walks through folders using classical mode.
2. User can pin a folder.
3. Internally, we either:
   a. initialize the folder as root by creating .ark subfolder in it
   b. or we mark it as favorite folder
4. If it's first root folder on the device, we enable ARK mode, too.
5. User can observe pinned folders in ARK tab.