[gui] improve no color logic by keeping RGB values

(cherry-picked from 64823d1)

Author: nyalldawson

src/gui/qgscolorbuttonv2.cpp

@@ -159,7 +159,9 @@ void QgsColorButtonV2::setToNoColor()
 {
   if ( mAllowAlpha )
   {
-    setColor( QColor( 0, 0, 0, 0 ) );
+    QColor noColor = QColor( mColor );
+    noColor.setAlpha( 0 );
+    setColor( noColor );
   }
 }