Order of mapping files in import configuration is not saved

[solth]

Bug description
As reported by @BartChris in #5219 the order of MappingFiles is not saved to the table importconfiguration_x_mappingfile. Therefore it is not guaranteed that the files are applied to the imported XML in the correct order.

To Reproduce
Steps to reproduce the behavior:

1. Create a new ImportConfiguration
2. Assign multiple mapping files to the new ImportConfiguration
3. Save the ImportConfiguration
4. Check the order of the mapping files in the database table importconfiguration_x_mappingfile

Expected behavior
The order of the MappingFiles should be explicitely saved to the one configured by the user in the Edit import configuration form.