Fix can't save custom colours in color picker (fix #15760)

Also fixes a memory leak caused by color picker widget not
being deleted

Author: nyalldawson

src/gui/qgscolorbuttonv2.cpp

@@ -400,6 +400,7 @@ void QgsColorButtonV2::panelAccepted( QgsPanelWidget* widget )
   if ( QgsCompoundColorWidget* colorWidget = qobject_cast< QgsCompoundColorWidget* >( widget ) )
   {
     addRecentColor( colorWidget->color() );
+    colorWidget->deleteLater();
   }
 }