Design page: https://community.kde.org/Kexi/Plugins/Queries/SQL_Constants
Bug: https://bugs.kde.org/show_bug.cgi?id=393094
[x] Base impl.
[x] Autotests for the SQL
[ ] Make conversion to QTime work for AM/PM - https://www.timeanddate.com/time/am-and-pm.html
[ ] Add error messages to date/time/date-time classes, otherwise SQL editor is unfriendly
[ ] Autotests for the date/time/date-time classes