Qt: Add missing multi-field column validation

Qt: Add missing multi-field column validation

GTK already has it, but Qt forgot about it, so multi-field custom column
works ok if previously saved in GTK-shark. Invalid validation prevent from
modifying and saving multi-field custom column in Qt version.

While at it, rename "custom field" to "custom fields" to ensure
we think about multi-field custom column.

Change-Id: I99588150ccb38be11b75f5dd5b0f6443e7055ebb
Reviewed-on: https://code.wireshark.org/review/12685
Petri-Dish: Michal Labedzki <michal.labedzki@tieto.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>