Restore support for date/time constants in KEXI SQL

Merge branch '393094-date-constants-3.2' into 3.2

- Add Date/Time classes for precise type support
- Add autotests for the KDb date/time classes
- Add support for the Date/Time constants in the SQL parser
- improve lexer too
- obsolete previous approach for SQL default date support
- and for SQLite, add new APIs for converting date/times

https://community.kde.org/Kexi/Plugins/Queries/SQL_Constants

- Add autotests for date/time related KDBSQL statements

BUG:393094

KDb VERSION -> 3.2.0 Beta 1

Test Plan:
Precondition: KDb branch: 393094-date-constants against 3.2 branch, KEXI branch 393094-date-constants (D17847)

0. Run KDb autotests
Expected: 100% passes

1. Open a new design in KEXI Query Designer.
Note: Visual designer does not support these constants, use the SQL view.

2. Type a number of statements involving Date, Time and Date/Time constants

You can use test cases from autotests/parser/data/statements.txt, sections:
- Date Constants (KDbSQL EXTENSION)
- Time Constants (KDbSQL EXTENSION)
- Date/Time Constants (KDbSQL EXTENSION)

Use the Check Query button to try both valid and invalid constants. For example "SELECT #12:13:01#" should work and "SELECT #2018-11-37#" should be rejected as invalid constant.

Expected: Valid constants should be accepted and then data view should work, there should be warning for invalid constants.
Expected: Saving of both valid and invalid statements should be possible in the SQL view.

{F6453115}

3. Create table with columns of type date, time and date/time and add some records.

4. Open a new design in Query SQL Designer, create statements involving both constants and columns
 e.g. SELECT #12:13:01# AS expr1, timecolumn FROM table where table.timecolumn < #2018-11-37#

Expected: valid statement, the Data view works.

{F6453113}

5. Test all of the above for SQLite, pgsql, mysql.

Differential Revision: https://phabricator.kde.org/D17336