Backup on save: Support time and date string replacements

Summary:
Supported variables are:
- %{Date:Locale}
- %{Date:ISO}
- %{Date:<value>} - see https://doc.qt.io/qt-5/qdate.html#toString
- %{Time:Locale}
- %{Time:ISO}
- %{Time:<value>} - see https://doc.qt.io/qt-5/qtime.html#toString
- %{ENV:<value>}
- %{JS:<expression>}
- %{UUID}

Screenshot:
{F7337486}

BUG: 403583
FIXED-IN: 5.63

Reviewers: cullmann

Reviewed By: cullmann

Subscribers: ngraham, kwrite-devel, kde-frameworks-devel

Tags: #kate, #frameworks

Differential Revision: https://phabricator.kde.org/D23832