Fix two-digit year formats issue: not allow by default

Summary: BUG:378481

Test Plan:
1. Run "LC_TIME=en_US kexi-3.2", open test db from https://bugs.kde.org/show_bug.cgi?id=369518

Expected:
Years are 4 digits now in tables and forms. On the output INFO message says as explained at https://bugs.kde.org/show_bug.cgi?id=378481#c5.
Data entry works based on this new year format. Changes are properly saved to db.
Display output e.g. in queries and reports follow this format too.

2. Set kexirc to
[General]
AllowTwoDigitYearFormats=true

And restart KEXI and open the same db.

Expected: original behavior: two-digit years.

Reviewers: piggz

Subscribers: Kexi-Devel-list

Tags: #kexi

Differential Revision: https://phabricator.kde.org/D18035