Improve ifcodegen qt_add_plugin integration

If you want build a static backend plugin a CLASS_NAME needs to be provided in the qt_add_plugin call.
This makes sure a _init library is created and that the plugin is imported when linked against the plugin.
As the plugin code is autogenerated and usually qt_ifcodegen_extend_target is used it's already too late to set the name from within the generator. Without setting the correct name the executable can't be linked because of undefined references.