Move "Import conversions" to importer

[koppor]

When importing a file, magic conversions take place:

I would prefer that these options are shown at each import and that one can change them per import. Or drop the completely and move the functionality to "CleanUp". Refs #60

The asking-at-each-time is implemented at the "link file" functionality:

[oscargus]

I agree. Also, if the current approach is used, it should also be used for "every" importer. In addition, it would make sense to generalize the clean-up dialog, such that each clean-up can be executed on arbitrary fields (or at least the conversion ones). In addition, features from #59 could be included.

[koppor]

Recent JabRef really behaves differently.

I think, we decided to move this to the save actions and thus this feature wish is obsolete.

JabRef even runs the save actions after an import.