Automatic discovery of ARK root folders

[kirillt]

Ensure that automatic discovery is moved from Navigator to this repo. All depending apps must trigger this process.

Example: on fresh Android installation, user installs ARK Shelf or ARK Memo. Then she attempts to select a folder for saving. Filepicker is invoked with 2 tabs: filesystem tab and roots tab. If the user selects roots tab and we are not aware of any roots registered, auto-discovery must happen and fill the tab with fresh list of ARK-enabled folders (a.k.a. root folders or ARK mounting points).

[kirillt]

This description is wrong, see newer issue:

• Discovery of ARK-enabled folders #21