Fix mapping selector in the batch import wizard to preserve stored values, even if not in the list of possible columns

Currently, in the import wizard, on the mapping selection step, if a value for a column is set and is not amongst the columns from the excel, its mapping is not preserved, is being wiped out.

This prevents features like constant and row index value mapping (BATCHIMP-7 and BATCHIMP-8) from working with the standard import wizard: the mappings, if stored in the batch import configuration class in the current document, are being cleaned everytime the mapping step of the wizard is passed.

Note that implementing this issue will still not add an UI for being able to manually map the row index or a constant, but will allow to preserve such mappings if they're already stored in a batch import config.