Remove usage of deprecated Qt API

Removes usage of the following deprecated Qt functions:
- QPixmap::grabWidget
- QString::null
- qFind
- qSort
- qGreater
- qCopy

src/core/SampleBuffer.cpp

@@ -1009,12 +1009,12 @@ QString SampleBuffer::openAudioFile() const
 	{
 		if( ofd.selectedFiles().isEmpty() )
 		{
-			return QString::null;
+			return QString();
 		}
 		return tryToMakeRelative( ofd.selectedFiles()[0] );
 	}
 
-	return QString::null;
+	return QString();
 }