Fix file index misdetection due to Unicode normalization issues

[jdpurcell]

This fixes an issue I noticed in the picture/video from @Andreas0602 in #531. The file number in the titlebar was showing as "0". I believe macOS is performing Unicode normalization when it passes the path to launch qView from Finder, whereas the files fetched from QDir::entryInfoList don't have the normalization. This confuses the optimization in updateLoadedIndexInFolder that does a QString::compare. The filenames are considered different because it's comparing a single "ä" character versus one composed of "a" + U+0308 (Combining Diaeresis). Note that the optimization is very important because in a folder with ~15K files, if it has to check all of them with FileInfo::operator== alone, it's taking ~1010ms on one of my test machines. The optimization allows it to run in ~1ms. I was able to fix this by adding some QString::normalize calls, which doesn't add noticeable overhead if the string doesn't end up changing, or very minor overhead even if all of those files have characters that result in normalization changes (completes in ~9ms overall).