normal mode: Importing only regular/basic fields from source file
advanced mode: Importing any related fields from source file(i.e. if detected pattern in header like '<relational_fieldname>/<fname>')
- Rename following checkbox "Show all fields for completion (advanced)" in "Show fields of relation fields (advanced)"
- If the import file contains a column with a field of a relation field, show the column label in the preview, even if "Show fields of relation fields (advanced)" is not checked > to make it less confusing when importing advanced files (e.g. products with attributes)
This is a backport of 1ba4fbe640, which was performed due to several
tickets mentioning confusion with the new import system.