if column name used for symbology changed, QGIS should ask when applying a QML file

[lutra]

Take a vector and apply some kind of styling to it, then save the QML style file.

Change the name of the column used for styling and try re-apply the QML file:

QGIS will fail to style the vector as in the QML file the column name is now wrong.

In this cases QGIS should warn the user and allow him/her to choose the column to be used to style the vector.