update FileWidget via signal to mark as not downloaded

[redshiftzero]

Right now when a file is deleted locally manually on the filesystem (i.e. not via the client), we do a full API sync, which will eventually update the UI. This will work, but we could update the UI immediately without waiting for a full sync. Instead I think we should:

• when we detect a file has been deleted, after marking it as not downloaded we emit a signal with that file's UUID
• a slot on FileWidget then updates the UI such that the "Download" button is displayed (if the UUID matches)

Followup from #566 (comment)

[sssoleileraaa]

Ah yes, I agree AND I think we should keep the local db synced with the local file system. Here's a little background: You can see where I added the change and why I added it. Basically, I think it makes sense to automatically look for other missing files if one file is detected missing in case there multiple files were moved because of some behind-the-scenes export or something.

I think we could still look for any missing file during a sync as well as immediately mark the file in the db as not downloaded at the time we try to export or open it (we'll need to remember there are two different places this happens when working on this ticket).

[redshiftzero]

I think we could still look for any missing file during a sync as well as immediately mark the file in the db as not downloaded at the time we try to export or open it (we'll need to remember there are two different places this happens when working on this ticket).

agreed! let's do this

[eloquence]

This was implemented; the client detects missing files both on sync and when opening/exporting them.