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] Make conversion to QTime work for AM/PM - https://www.timeanddate.com/time/am-and-pm.html#converting
[x] Add autotests for the date SQL constants
[x] Add autotests for the time SQL constants
[ ] Add autotests for the date/date SQL constants
[x] Add simple autotests for the date/time/date-time classes
[ ] Add complete autotests for the date/time/date-time classes (corner cases, etc.)
[ ] Add error messages to date/time/date-time classes, otherwise SQL editor is unfriendly