validate layout settings

If you create a panels template with layout settings, there's no way to validate those settings.

includes/display_edit.inc has a function panels_edit_display_settings_form_validate which looks for a validate function in the template and calls it, but panels_edit_display_settings_form_validate doesn't appear to ever get called.

The attached patch fixes it by adding validate functions to the wrapped forms which in turn call panels_edit_display_settings_form_validate, though I'm not sure if this is the right approach.