GrailsTypeRenameWizard: rename similarly named bits and pieces in Controller

In particular, Grails generated controller tend to use parameters and map keys called 'songInstance' if the corresponding domain class is called 'Song'.

While not renaming these 'songInstance' elements when renaming the class doesn't seem to break the code, it does seem odd to have something called 'songInstance' when the class is no longer called 'Song'.