Improve the "File columns" selection in the "Test with file" function

[Nicklas Nordborg]

The "File columns" is a set of regular drop-down <select> elements in the dialog where the "Test with file" functionality is mapping columns from the file to properties on the imported items.

I think the selection list is a bit user-unfriendly. For example it is not always obvious which column in the file that contains the data that should be mapped to a property. A user may have to switch back and forth between the "File data" and "Column mapping" tabs to determine the exact columns to use.

An enhancement could for example be to display example data from the first data line in the selection list in addition to the column header.

[Nicklas Nordborg]

In 8132:

Fixes #2297: Improve the "File columns" selection in the "Test with file" function