Discovery of ARK-enabled folders

[kirillt]

Similar to how it's done in Navigator:

• Automatic root folders discovery and favorite folders persistence  ARK-Navigator#221

An ARK-enabled folder is either:

• a root folder—any folder containing .ark subfolder
• a favorite folder—subfolder of a root mentioned in .ark/favorites file

The discovery can be triggered when there is no .ark-roots file in the disk root and we can't know if there are any root folder or no. But also it can be triggered to discover new manually synced folders.

This can be useful in 2 scenarios:

• Folder pinning #20
• Discovery of manually synced ARK folders #22

Questions:
1. Should we allow the user scanning filesystem manually?
Can be represented as "refresh" button in ARK mode.

2. Should we perform the scanning automatically and regularly without asking the user?
Probably should be configurable.