Recursively show status of sync on folders with overlay icon

### Is your feature request related to a problem? Please describe.
It is not visible which files might require synchronization and which not, and where they might be. Especially due to issue #2753 it is quite cumbersome to do too often syncs on big folders so that knowing where missing files might be would be very helpful.

### Describe the solution you'd like
Having an overlay icon (the downloaded green arrow or the red cross) on folders like on files, showing recursively if an unsynced file is waiting in the hierarchy to be synced.

### Describe alternatives you've considered
Like there is "All files" and "On Device" have an "Unsynced" view with only the files requiring synchronization. Actually both approaches could be implemented independently from each other.