Issue 6121 : Extend system strings with date/time to be customized by user.

ADDED: NOW, LAST_UPDATE, YEAR, MONTH, DAY_NBR (day of month), DAY (Monday, Tuesday...), HOUR, MINUTE and SECOND text vars. These system strings may be used in schematics and boards. MONTH and other vars enable customization of date/time, inside a string defined by user. As an example, the user can define a string : "Today is ${DAY} ${MONTH} ${DAY_NBR}". It will be resolved as "Today is Thursday July 28".

NOW is updated when schematic/board is displayed or saved. Other vars are updated when schematic/board is saved. All data are saved as text vars in project file (Text vars setting).

Other strings can be easily added in code.

See: #6121