Date and timestamp values are not exported correctly with the table exporter

[Nicklas Nordborg]

It seems like in Excel they are always exported as strings and they are not using the configured preferences of the user. Example:

'2016-07-04	Mon Jul 04 13:30:00 CEST 2016

I think this may be a side-effect of the fix for #2336 which fixed a similar issue for numeric values, but probably made it worse for date and timestamp values.

[Nicklas Nordborg]

In 8333:

Fixes #2344: Date and timestamp values are not exported correctly with the table exporter

[Nicklas Nordborg]

Now the thousands separator setting is included for numeric values when exporting to CSV format.

[Nicklas Nordborg]

In 8334:

References #2344: Date and timestamp values are not exported correctly with the table exporter

This was a bit more complicated than expected. There are now some more alternatives depending on the value type and if the export is going to Excel or CSV.

[Nicklas Nordborg]

In 8335:

References #2344: Date and timestamp values are not exported correctly with the table exporter

Remove unsused import.