Do not crash on invalid EXPORTER_CLASSES settings when using FORM_DESIGN...

...ER_WIDGET_CLASSES

Currently you cannot use FORM_DESIGNER_WIDGET_CLASSES inside your own settings file because this information is also used to overwrite the EXPORTER_CLASSES.

I fixed it by using a separate setting, FORM_DESIGNER_EXPORTER_CLASSES, for overwriting the EXPORTER_CLASSES variable.