[osx] Fix missing overlay icons on client startup

[osx] Fix missing overlay icons on client startup

Since the statuses are cached and that we can't invalidate the cache,
sending NOP would need to be overwritten by the default OK status
once the client successfully connected. But instead of remembering
which files we NOPed, rather wait until we are ready to sync before
sending the REGISTER_PATH message to the socket API client. It will
also prevent the client from sending unnecessary RETRIEVE_FILE_STATUS
requests.

Also remove AccountState::canSync, since it does the same as
isConnected and syncing is not an account responsibility.