Fixed a problem with the import of .csv-files

Up until now, the DBBrowserDB:decodeCSV used to load the csv-file character by character using the getChar() method of the QFile-class.
Unfortunatelly, this approach caused multibyte-chars as used for UTF-8 encoding to be split and displayed incorrectly.

The fix uses the QTextStream-class to load the file line by line.
Every line is then again iterated character by character, before the old algorithm is applied. Using this approach, the characters are loaded and encoded properly. Splitting multibyte-chars is thus prevented.