This also remove all smartness from the SocketApi about the status
of a file and solely use info from the current and last sync.
This simplifies the logic a lot and prevents any discrepancy between
the status shown in the activity log and the one displayed on the
overlay icon of a file.
The main benefit of the additional simplicity is that we are able
to push all new status of a file reliably (including warnings for
parent folders) to properly update the icon on overlay implementations
that don't allow us invalidating the status cache, like on OS X.
Both errors and warning from the last sync are now kept in a set,
which is used to also affect parent folders of an error.
To make sure that errors don't become warning icons on a second
sync, SyncFileItem::_hasBlacklistEntry is also interpreted as an error.
This also renames StatusIgnore to StatusWarning to match this semantic.
SyncEngine::aboutToPropagate is used in favor of SyncEngine::syncItemDiscovered
since the latter is emitted before file permission warnings are set on the
SyncFileItem. SyncEngine::finished is not used since we have all the
needed information in SyncEngine::itemCompleted.
SyncFileStatus' purpose is to track overlay icon status.
Instead of putting comments and default: clauses in switch
on both sides about unused enums, use different enums.
This also remove STATUS_NEW which is the equivalent of
STATUS_SYNC in all shell extension implementations, and
remove STATUS_UPDATED and STATUS_STAT_ERROR which have
the same semantic as STATUS_UPTODATE and STATUS__ERROR.
