Fix the root item sync status #4682

Make sure that we push the new status when the status of the SyncEngine
changed. SyncEngine::started comes a bit late, only when the propagation
starts, although it's better in this case since child folders will
only switch to Sync in aboutToPropagate.

Also fix an issue with SyncEngine::findSyncItem when using an empty
fileName; this would match and return the wrong item, even though
not currently happening with the code since fileStatus won't call
it with an empty fileName anymore.