Add basic support for syncing files.

What’s not supported yet:
* Setting a custom style and syncing the actual name between devices. You have to let files sync then select them on each device.

What’s supported:
* Placing a style on iCloud and having it available on other devices.

iCloud folder can be reached by opening the add-ons folder from the Installed Addons section of Preferences when cloud syncing is enabled.
Conditional fail