Some improvements for the file dialog. There is still a big latency the
first time a directory is loaded, we have to optimise the loop in
filterAcceptsRow(), ie.:
SequenceParsing::FileNameContent
fileContent(absoluteFilePath.toStdString());
    for (int i = (int)_frameSequences.size() - 1; i >= 0 ; --i) {
        if (_frameSequences[i]->tryInsertFile(fileContent,false)) {
            ///don't accept the file in the proxy because it already
belongs to a sequence
            _filterCache.insert(std::make_pair(absoluteFilePath,false));
            return false;
        }
    }