OSO: if the media is in Inbox, don't write there

The Inbox folder is RO for app created content. Only deleting items stored there by the OS is permitted. Trying to save an external subtitles file there will silently fail and never make the item available to the user.

Fixes #1326 (closed) and #1327 (closed).

This MR also implements a caching mechanism for subtitles downloaded for the affected media by storing them in a designated location and performing a lookup if needed when playback starts (again).