[File browser] Keep directory structure

[ferndot]

Instead of enumerating all editable files in a storage into a list, we should preserve the directory structure. This will make the app better suited to large storages filled with editable files.

[ferndot]

Depends on the fix in #181

[twiss]

Re this fixing #218, I don't think there's a way to list one level in the Device Storage API, so I'm guessing you simply want to lazily put the documents in the DOM one level at a time?

[ferndot]

There is a path parameter that can be used with DeviceStorage.enumerate() to limit the results to a specific directory (https://developer.mozilla.org/docs/Web/API/DeviceStorage.enumerate). Unfortunately, I just checked, and there is no option to disable deep enumeration (directory and its sub-directories). I will file a bug.

[Ryuno-Ki]

Already done. See my reference in #218.