[overlays] Fix folders appearing as OK even though children are still syncing #4797

[overlays] Fix folders appearing as OK even though children are still syncing #4797

This would happen if the directory would first need to be created
through an mkdir propagation job. This job's itemCompleted signal
would trigger the directory to show as SYNC even though its children
are still propagating.

Fix the issue by tracking the sync count for each file, affecting
its parents. This allows us to get rid of the O(n) vector lookup
for each status query, and properly track the hierachical sync
status of a directory.

This also removes the itemCompleted signal emission from the
PropagateDirectory job. Since we only needed for overlay icons, and
since this job doesn't do any direct propagation, we can remove it
to ensure that we won't call itemCompleted twice for the item attached
to Propagate*Mkdir jobs (since the PropagateDirectory is backed by
the same SyncFileItem, instruction and status).