Fix non-existent 'UTF-8-BOM' to 'UTF-8'

I am using KDiff to diff files on Windows that are UTF-8 with BOM. With the current version of KDiff, a few garbage symbols displayed at the start of every file. I have traced this to calling QTextCodec::codecForName("UTF-8-BOM"), which always returns null because "UTF-8-BOM" appears to not be a recognized codec name. Instead, we should call QTextCodec::codecForName("UTF-8"). Making this change fixed it so that the garbage symbols no longer display at the start of the file.